[New AHA and ACC guidelines on the treatment of blood cholesterol to reduce atherosclerotic cardiovascular risk : Statement of the D•A•CH Society for Prevention of Cardiovascular Diseases, the Austrian Atherosclerosis Society and the Working Group on Lipids and Atherosclerosis (AGLA) of the Swiss Society for Cardiology]

Guidelines for the reduction of cholesterol to prevent atherosclerotic vascular events were recently released by the American Heart Association and the American College of Cardiology. The authors claim to refer entirely to evidence from randomized controlled trials, thereby confining their guidelines to statins as the primary therapeutic option. The guidelines derived from these trials do not specify treatment goals, but refer to the percentage of cholesterol reduction by statin medication with low, moderate, and high intensity. However, these targets are just as little tested in randomized trials as are the cholesterol goals derived from clinical experience. The same applies to the guidelines of the four patient groups which are defined by vascular risk. No major statin trial has included patients on the basis of their global risk; thus the allocation criteria are also arbitrarily chosen. These would actually lead to a significant increase in the number of patients to be treated with high or maximum dosages of statins. Also, adhering to dosage regulations instead of cholesterol goals contradicts the principles of individualized patient care. The option of the new risk score to calculate lifetime risk up to the age of 80 years in addition to the 10-year risk can be appreciated. Unfortunately it is not considered in the therapeutic recommendations provided, despite evidence from population and genetic studies showing that even a moderate lifetime reduction of low-density lipoprotein (LDL) cholesterol or non-HDL cholesterol has a much stronger effect than an aggressive treatment at an advanced age. In respect to secondary prevention, the new American guidelines broadly match the European guidelines. Thus, the involved societies from Germany, Austria and Switzerland recommend continuing according to established standards, such as the EAS/ESC guidelines.

Afamin stimulates osteoclastogenesis and bone resorption via Gi-coupled receptor and Ca2+/calmodulin-dependent protein kinase (CaMK) pathways

BACKGROUND

Afamin was recently identified as a novel osteoclast-derived coupling factor that can stimulate the in vitro and in vivo migration of preosteoblasts.

AIM

In order to understand in more detail the biological roles of afamin in bone metabolism, we investigated its effects on osteoclastic differentiation and bone resorption.

METHODS

Osteoclasts were differentiated from mouse bone marrow cells. Tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells were considered as osteoclasts, and the resorption area was determined by incubating the cells on dentine discs. The intracellular cAMP level was determined using a direct enzyme immunoassay. Signaling pathways were investigated using western blot and RT-PCR. Recombinant afamin was administered exogenously to bone cell cultures.

RESULTS

Afamin stimulated both osteoclastogenesis and in vitro bone resorption. Consistently, the expressions of osteoclast differentiation markers were significantly increased by afamin. Although afamin mainly affected the late-differentiation stages of osteoclastogenesis, the expression levels of receptor activator of nuclear factor-κB ligand (RANKL)-dependent signals were not changed. Afamin markedly decreased the levels of intracellular cAMP with reversal by pretreatment with pertussis toxin (PTX), a specific inhibitor of Gi-coupled receptor signaling. In addition, PTX almost completely blocked afamin-stimulated osteoclastogenesis. Furthermore, pretreatment with KN93 and STO609 - Ca2+/cal - mo dulin-dependent protein kinase (CaMK) and CaMK kinase inhibitors, respectively - significantly prevented decreases in the intracellular cAMP level by afamin while attenuating afamin-stimulated osteoclastogenesis.

CONCLUSION

Afamin enhances osteoclastogenesis by decreasing intracellular cAMP levels via Gi-coupled receptor and CaMK pathways.

The satiety factor apolipoprotein A-IV modulates intestinal epithelial permeability through its interaction with alpha-catenin: implications for inflammatory bowel diseases

INTRODUCTION

Apolipoprotein A-IV (apoA-IV), an intestinally and cerebrally synthesized satiety factor and anti-atherogenic plasma apolipoprotein, was recently identified as an anti-inflammatory protein. In order to elucidate whether intestinal apoA-IV exerts similar repair function as its hepatic homologue apolipoprotein A-V (apoA-V), apoA-IV-interactive proteins were searched and in vitro functional studies were performed with apoA-IV overexpressing cells. ApoA-IV was also analyzed in the intestinal mucosa of patients with inflammatory bowel diseases (IBD), together with other genes involved in epithelial junctional integrity.

METHODS

A yeast-two-hybrid screening was used to identify apoA-IV-interactors. ApoA-IV was overexpressed in Caco-2 and HT-29 mucosal cells for colocalization and in vitro epithelial permeability studies. Mucosal biopsies from quiescent regions of colon transversum and terminal ileum were subjected to DNA-microarray analysis and pathway-related data mining.

RESULTS

Four proteins interacting with apoA-IV were identified, including apolipoprotein B-100, alpha1-antichymotrypsin, cyclin C, and the cytosolic adaptor alpha-catenin, thus linking apoA-IV to adherens junctions. Overexpression of apoA-IV was paralleled with a differentiated phenotype of intestinal epithelial cells, upregulation of junctional proteins, and decreased paracellular permeability. Colocalization between alpha-catenin and apoA-IV occurred exclusively in junctional complexes. ApoA-IV was downregulated in quiescent mucosal tissues from patients suffering from IBD. In parallel, only a distinct set of junctional genes was dysregulated in non-inflamed regions of IBD gut.

CONCLUSIONS

ApoA-IV may act as a stabilizer of adherens junctions interacting with alpha-catenin, and is likely involved in the maintenance of junctional integrity. ApoA-IV expression is significantly impaired in IBD mucosa, even in non-inflamed regions.

In vivo turnover study demonstrates diminished clearance of lipoprotein(a) in hemodialysis patients

Lipoprotein(a) (Lp(a)) consists of a low-density lipoprotein-like particle and a covalently linked highly glycosylated protein, called apolipoprotein(a) (apo(a)). Lp(a) derives from the liver but its catabolism is still poorly understood. Plasma concentrations of this highly atherogenic lipoprotein are elevated in hemodialysis (HD) patients, suggesting the kidney to be involved in Lp(a) catabolism. We therefore compared the in vivo turnover rates of both protein components from Lp(a) (i.e. apo(a) and apoB) determined by stable-isotope technology in seven HD patients with those of nine healthy controls. The fractional catabolic rate (FCR) of Lp(a)-apo(a) was significantly lower in HD patients compared with controls (0.164+/-0.114 vs 0.246+/-0.067 days(-1), P=0.042). The same was true for the FCR of Lp(a)-apoB (0.129+/-0.097 vs 0.299+/-0.142 days(-1), P=0.005). This resulted in a much longer residence time of 8.9 days for Lp(a)-apo(a) and 12.9 days for Lp(a)-apoB in HD patients compared with controls (4.4 and 3.9 days, respectively). The production rates of apo(a) and apoB from Lp(a) did not differ significantly between patients and controls and were even lower for patients when compared with controls with similar Lp(a) plasma concentrations. This in vivo turnover study is a further crucial step in understanding the mechanism of Lp(a) catabolism: the loss of renal function in HD patients causes elevated Lp(a) plasma levels because of decreased clearance but not increased production of Lp(a). The prolonged retention time of Lp(a) in HD patients might importantly contribute to the high risk of atherosclerosis in these patients.

Vitamin E binding protein afamin protects neuronal cells in vitro

Afamin, an 87 kDa human plasma glycoprotein with specific binding properties for vitamin E (alpha-tocopherol) was recently characterized (Jerkovic, 1997; Vögele, 1999). In the present study the in vitro effects on neuronal cells of native human Afamin, of Afamin pre-loaded with vitamin E (Afamin+), and of vitamin E were investigated. Isolated cortical chicken neurons were maintained either under apoptosis-inducing low serum conditions or exposed to oxidative stress by the addition of H2O2 or beta-amyloid peptide(25-35). Afamin and vitamin E synergistically enhance the survival of cortical neurons under apoptotic conditions. Furthermore, Afamin alone protects cortical neurons from cell death in both experimental settings. Therefore, the plasma glycoprotein Afamin apparently displays a neuroprotective activity not only by virtue of binding and transporting vitamin E but also on its own.

[Genetic-epidemiological studies of apolipoprotein(a) polymorphism and its significance in nephrological diseases and type I diabetes mellitus]

Lipoprotein(a) is a highly atherogenic particle. The plasma concentrations of this lipoprotein are strongly related to a genetically determined size polymorphism of apolipoprotein(a). This article reviews some pathogenetic characteristics of the apolipoprotein(a) polymorphism besides its known effect on the lipoprotein(a) plasma concentrations. Those are the relation of the apolipoprotein(a) phenotype with atherogenesis, the apolipoprotein(a) phenotype-specific elevation of lipoprotein(a) in hemodialysis patients and the advantages of this polymorphism for the atherosclerosis risk evaluation in high-risk patients. It furthermore discusses the observed association between the low molecular weight apolipoprotein(a) phenotype and Type I diabetes mellitus.

Renal handling of human apolipoprotein(a) and its fragments in the rat

The sites and mechanisms of the catabolism of atherogenic lipoprotein(a) (Lp(a)) are not well understood. Lp(a) is increased in patients with end-stage renal disease, suggesting a renal catabolism of Lp(a). To gain a better insight into renal handling of Lp(a), we established a heterologous rat model to study the renal catabolism of human Lp(a). Pure human Lp(a) was injected into Wistar rats, and animals were sacrificed at different time points (30 minutes to 24 hours). Intact Lp(a) was cleared from the circulation of injected rats with a half-life time of 14.5 hours. Strong intracellular immunostaining for apolipoprotein(a) (apo(a)) was observed in the cytoplasm of proximal tubular cells after 4, 8, and 24 hours. Apolipoprotein B (apoB) was colocalized with glomerular apo(a) 1 to 8 hours after Lp(a) injection, but renal capillaries and tubules remained negative. No relevant amounts of apo(a) fragments were found in the plasma of rats after injection of Lp(a). During all urine collection periods, apo(a) fragments with molecular weights of 50 to 160 kd were detected in the urine, however. Our results show that human Lp(a) injected into rats accumulates intracellularly in the rat kidney, and apo(a) fragments are excreted in the urine. The kidney apparently plays a major role in fragmentation of Lp(a). Despite the fact that rodents lack endogenous Lp(a), rats injected with human Lp(a) may provide a useful heterologous animal model to study the renal metabolism of Lp(a) further.

The association of serum lipoprotein(a) levels, apolipoprotein(a) size and (TTTTA)(n) polymorphism with coronary heart disease

BACKGROUND

The association between lipoprotein(a) levels, apolipoprotein(a) size and the (TTTTA)(n) polymorphism which is located in the 5' non-coding region of the apo(a) gene was studied in 263 patients with severe coronary heart disease and 97 healthy subjects.

METHODS

Lp(a) levels were measured by ELISA, apo(a) isoform size was determined by SDS-agarose gel electrophoresis, and analysis of the (TTTTA)(n) was carried out by PCR. For statistical calculation, both groups were divided into low (at least one apo(a) isoform with < or = 22 Kringle IV) and high (both isoforms with >22 KIV) apo(a) isoform sizes, and into low number (<10 in both alleles) and high number of (> or =10 at least one allele) TTTTA repeats.

RESULTS

Lp(a) levels were higher (P=0.007), apo(a) isoforms size < or =22 KIV and TTTTA repeats > or = 10 were more frequent (P=0.007 and 0.01) in cases than in controls. Lp(a) levels were found to be increased with low apo(a) weight in both groups (both P<0.0001). In multivariate logistic regression analysis, only the Lp(a) levels (P=0.005) and (TTTTA)(n) polymorphism (P=0.002) were found to be significantly associated with CHD.

CONCLUSION

Nevertheless, these results indicate that in CHD patients the (TTTTA)(n) polymorphism has an effect on Lp(a) levels which is independent of the apo(a) size.

Low apolipoprotein A-IV plasma concentrations in men with coronary artery disease

OBJECTIVES

The objective of this study was to evaluate the relation between apolipoprotein A-IV (apoA-IV) plasma concentrations and coronary artery disease (CAD).

BACKGROUND

Experimental in vitro and in vivo studies favor apoA-IV to be protective against the development of atherosclerosis. Mice that overexpress either human or mouse apoA-IV demonstrated a significant reduction of aortic atherosclerotic lesions compared with control mice. Data on apoA-IV plasma concentrations and CAD in humans are lacking.

METHODS

We determined in two independent case-control studies of a Caucasian and an Asian Indian population whether apoA-IV plasma concentrations are related to the presence of angiographically assessed CAD.

RESULTS

Plasma apoA-IV levels were significantly lower in 114 male Caucasian subjects with angiographically defined CAD when compared with 114 age-adjusted male controls (10.2 +/-3.8 mg/dL vs. 15.1 +/- 4.0 mg/dL, p < 0.001). Logistic regression analysis indicated that the association between apoA-IV levels and CAD was independent of the high-density lipoprotein cholesterol and triglyceride concentrations. The inverse relationship between plasma levels of apoA-IV and the presence of CAD was confirmed in an independent sample of 68 male Asian Indians with angiographically documented CAD and 68 age-matched controls.

CONCLUSIONS

The results of this cross-sectional study demonstrate for the first time an association between low apoA-IV concentrations and CAD in humans and suggest that apoA-IV may play an antiatherogenic role in humans.

Preparative free-solution isotachophoresis for separation of human plasma lipoproteins: apolipoprotein and lipid composition of HDL subfractions

We have previously shown that plasma lipoproteins can be separated by analytical capillary isotachophoresis (ITP) according to their electrophoretic mobility in a defined buffer system. As in lipoprotein electrophoresis, HDL show the highest mobility followed by VLDL, IDL, and LDL. Chylomicrons migrate according to their net-charge between HDL and VLDL, because ITP has negligible molecular sieve effects. Three HDL subfractions were obtained which were designated fast-, intermediate-, and slow-migrating HDL. To further characterize these HDL subfractions, a newly developed free-solution ITP (FS-ITP)-system was used, that allows micro-preparative separation of human lipoproteins directly from whole plasma (Böttcher, A. et al. 1998. Electrophoresis. 19: 1110-1116). The fractions obtained by FS-ITP were analyzed for their lipid and apolipoprotein composition and by two-dimensional nondenaturing polyacrylamide gradient gel electrophoresis (2D-GGE) with subsequent immunoblotting. fHDL are characterized by the highest proportion of esterified cholesterol of all three subfractions and are relatively enriched in LpA-I. Together with iHDL they contain the majority of plasma apoA-I, while sHDL contain the majority of plasma apoA-IV, apoD, apoE, and apoJ. Pre-beta-HDL were found in separate fractions together with triglyceride-rich fractions between sHDL and LDL. In summary, ITP can separate the bulk of HDL into lipoprotein subfractions, which differ in apolipoprotein composition and electrophoretic mobility. While analytical ITP permits rapid separation and quantitation for diagnostic purposes, FS-ITP can be used to obtain these lipoprotein subfractions on a preparative scale for functional analysis. As FS-ITP is much better suited for preparative purposes than gel electrophoresis, it represents an important novel tool for the functional analysis of lipoprotein subclasses.

Is apolipoprotein(a) a susceptibility gene for type I diabetes mellitus and related to long-term survival?

AIMS/HYPOTHESIS

High lipoprotein(a) [Lp(a)] plasma concentrations are a genetically determined risk factor for atherosclerotic complications. In healthy subjects Lp(a) concentrations are mostly controlled by the apolipoprotein(a) [apo(a)] gene locus which determines a size polymorphism with more than 30 alleles. Subjects with low molecular weight apo(a) phenotypes on average have higher Lp(a) concentrations than those with high molecular weight apo(a) phenotypes. There are many opinions about whether and why Lp(a) is raised in patients with Type I diabetes (insulin-dependent) mellitus.

METHODS

We investigated Lp(a) plasma concentrations and apo(a) phenotypes in 327 patients with Type I diabetes mellitus (disease duration 1-61 years) and in 200 control subjects matched for age and sex.

RESULTS

Patients with a disease duration of up to 15 years had significantly higher Lp(a) concentrations (24.3 +/- 34.0 mg/dl vs 16.7 +/- 22.6 mg/dl, p = 0.014) compared with control subjects. This increase can be explained by a considerably higher frequency of low molecular weight apo(a) phenotypes (38.9% vs 23.5%, p < 0.005). The frequency of low molecular weight apo(a) phenotypes decreased continuously with disease duration from 41.7% in those with disease duration of up to 5 years to 18.2% in those with the disease lasting more than 35 years.

CONCLUSIONS/INTERPRETATION

Our data show that an increase of Lp(a) in Type I diabetic patients can only be observed in groups with short diabetes duration and that this elevation is genetically determined. Therefore, the apo(a) gene, located at 6q26-27, might be a susceptibility gene for Type I diabetes mellitus which is supported by recently published studies reporting evidence for linkage of this region (6q27) with Type I diabetes mellitus. Furthermore, the decreasing frequency of low molecular weight apo(a) phenotypes with disease duration suggests a survivor effect.

European Lipoprotein Club: report of the 21st Annual Conference, Tutzing, September 28-October 1, 1998

Molecular biology and genetics were the hallmarks of the conference. Attendees from 20 European countries participated in lively discussions with international speakers. The opening round table session entitled 'Genetic approach to complex diseases' was chaired by Harald Funke. Steve Humphries (London) presented association studies and Harald Funke (Munster) presented multiparameter analyses, as models of genetic epidemiological approaches to atherosclerosis. Gerd Utermann (Innsbruck) showed, through sib pair linkage analysis, how apo (a) gene polymorphism determines plasma levels of Lp(a). Klaus Lindpainter (Basel) described novel genetic strategies heading for a more targeted medicine, through the identification of genetic mechanisms of disease and therapeutic responses. Session I, chaired by Richard James (Geneva) and Guido Franceschini (Milano), on 'Basic mechanisms of action of drugs' highlighted molecular and cellular actions by which present (fibrates, statins) or future (ACAT or MTP inhibitors) drugs or hormones may modulate lipoprotein metabolism. Marten Hofker (Leiden) and Philippa Talmud (London) chaired Session II on 'Regulation of gene expression', which reported cellular regulations by nuclear receptors (PPARs), or the regulation of lipid trafficking by membrane receptors (SR-BI, Megalin, Apo-E receptor, scavenger receptors) or by intracellular (IFN gamma signalling pathways) or extracellular proteins (lipases). Beyond gene expression, Session III, 1st part, entitled 'Lipoprotein modifying enzymes' was chaired by Katriina Aalto-Setälä (Tampere). Roles of lipases (HL, LPL) and transfer proteins (CETP, PLTP), as well as structures of lipid binding molecules (LCAT, apolipoproteins), were further explored. The 'Gene interactions' session chaired by Rudolph Poledne (Prague), and 'Novelties' chaired by Hans Dieplinger (Innsbruck), reported elegant models of cross-bred, tissue specific knock-out or YAC-transgenic mice for lipoprotein metabolism, and descriptions of gene interactions in polygenic disorders or new loci for familial lipid disorders (familial combined hyperlipidemia, metabolic syndrome and Tangier disease) in humans.

Lipoprotein(a) plasma concentrations after renal transplantation: a prospective evaluation after 4 years of follow-up

The highly atherogenic lipoprotein(a) [Lp(a)] is significantly elevated in patients with renal disease. It is discussed controversially whether Lp(a) concentrations decrease after renal transplantation and whether the mode of immunosuppressive therapy influences the Lp(a) concentrations. In a prospective study the Lp(a) concentrations before and on average 48 months after renal transplantation were measured in 145 patients. The determinants of the relative changes of Lp(a) concentrations were investigated in a multivariate analysis. Patients treated by CAPD showed a larger decrease of Lp(a) than hemodialysis patients, reflecting their markedly higher Lp(a) levels before transplantation. The relative decrease of Lp(a) was higher with increasing Lp(a) concentrations before transplantation in combination with an increasing molecular weight of apolipoprotein(a) [apo(a)]. That means that the relative decrease of Lp(a) is related to the Lp(a) concentration and the apo(a) size polymorphism. With increasing proteinuria and decreasing glomerular filtration rate, the relative decrease of Lp(a) became less pronounced. Neither prednisolone nor cyclosporine (CsA) had a significant impact on the Lp(a) concentration changes. Azathioprine (Aza) was the only immunosuppressive drug which had a dose-dependent influence on the relative decrease of Lp(a) levels. These data clearly demonstrate a decrease of Lp(a) following renal transplantation which is caused by the restoration of kidney function. The relative decrease is influenced by Aza but not by CsA or prednisolone.

The seventh myth of lipoprotein(a): where and how is it assembled?

Our understanding of the genetics, metabolism and pathophysiology of the atherogenic plasma lipoprotein Lp(a) has considerably increased over past years. Nevertheless, the precise mechanisms regulating the biosynthesis and assembly of Lp(a) are poorly understood and controversially discussed. Lp(a) plasma concentrations are determined by synthesis and not by degradation. Transcriptional and post-translational mechanisms have been identified as regulating Lp(a) production in primary hepatocytes and transfected cell lines. Assembly of Lp(a) occurs extracellularly from newly synthesized apolipoprotein(a) and circulating LDL. This view has recently been challenged by in-vivo kinetic studies in humans which are compatible with an intracellular assembly event. Lp(a) assembly is a complex two-step process of multiple non-covalent interactions between apolipoprotein(a) and apolipoprotein B-100 of LDL followed by covalent disulfide linkage of two free cysteine residues on both proteins.

The low molecular weight apo(a) phenotype is an independent predictor for coronary artery disease in hemodialysis patients: a prospective follow-up

Patients with end-stage renal disease treated by hemodialysis have a tremendous risk for cardiovascular complications that cannot be explained by traditional atherosclerosis risk factors. Lipoprotein(a) (Lp(a)), a risk factor for these complications in the general population, is significantly elevated in these patients. In this study, it was determined whether Lp(a) and/or the genetically determined apo(a) phenotype are risk predictors for the development of coronary artery disease in these patients. A cohort of 440 unselected hemodialysis patients were followed for a period of 5 yr independent of the cause of renal disease, duration of preceding treatment, and the preexistence of coronary artery disease at study entry. Coronary events defined as definite myocardial infarction, percutaneous transluminal coronary angioplasty, aortocoronary bypass, or a stenosis >50% in the coronary angiography were the main outcome measure. Sixty-six (15%) of the 440 patients suffered a coronary event during follow-up. In univariate analysis, patients with events were significantly older and showed a trend to lower HDL cholesterol concentrations, and higher apolipoprotein B and Lp(a) concentrations without reaching significance. Apo(a) phenotypes of low molecular weight, however, were significantly more frequent in patients with compared to those without events (43.9% versus 21.9%, P<0.001). The other lipids, lipoproteins, and apolipoproteins were similar in both groups. Multiple Cox proportional hazards regression analysis found age and the apo(a) phenotype to be the best predictors for coronary events during the observation period, independent of whether patients with a preexisting coronary artery disease or an age >65 yr at the study entry or both were excluded from the analysis. Diabetes mellitus was a risk factor only in presence of a low molecular weight apo(a) phenotype. The genetically determined apo(a) phenotype is a strong and independent predictor for coronary events in hemodialysis patients. Apo(a) phenotyping might be helpful to identify hemodialysis patients at high risk for coronary artery disease.

Identification of megalin/gp330 as a receptor for lipoprotein(a) in vitro

Lipoprotein(a) [Lp(a)] is an atherogenic lipoprotein of unknown physiological function. The mechanism of Lp(a) atherogenicity as well as its catabolic pathways are only incompletely understood at present. In this report, we show that the low density lipoprotein receptor (LDLR) gene family member megalin/glycoprotein (gp) 330 is capable of binding and mediating the cellular uptake and degradation of Lp(a) in vitro. A mouse embryonic yolk sac cell line with native expression of megalin/gp330 but genetically deficient in LDLR-related protein (LRP) and a control cell line carrying a double knockout for both LRP and megalin/gp330 were compared with regard to their ability to bind, internalize, and degrade dioctadecyltetramethylindocarbocyanine perchlorate (DiI)-fluorescence-labeled Lp(a) as well as equimolar amounts of 125I-labeled Lp(a) and LDL. Uptake and degradation of radiolabeled Lp(a) by the megalin/gp330-expressing cells were, on average, 2-fold higher than that of control cells. This difference could be completely abolished by addition of the receptor-associated protein, an inhibitor of ligand binding to megalin/gp330. Mutual suppression of the uptake of 125I-Lp(a) and of 125I-LDL by both unlabeled Lp(a) and LDL suggested that Lp(a) uptake is mediated at least partially by apolipoprotein B100. Binding and uptake of DiI-Lp(a) resulted in strong signals on megalin/gp330-expressing cells versus background only on control cells. In addition, we show that purified megalin/gp330, immobilized on a sensor chip, directly binds Lp(a) in a Ca2+-dependent manner with an affinity similar to that for LDL. We conclude that megalin/gp330 binds Lp(a) in vitro and is capable of mediating its cellular uptake and degradation.

Genetics and metabolism of lipoprotein(a) and their clinical implications (Part 1)

The human plasma lipoprotein Lp(a) has gained considerable clinical interest as a genetically determined risk factor for atherosclerotic vascular diseases. Numerous (including prospective) studies have described a correlation between elevated Lp(a) plasma levels and coronary heart disease, stroke and peripheral atherosclerosis. Lp(a) consists of a large LDL-like particle to which the specific glycoprotein apo(a) is covalently linked. The apo(a) gene is located on chromosome 6 and belongs to a gene family including the highly homologous plasminogen. Lp(a) plasma concentrations are controlled to a large extent by the extremely polymorphic apo(a) gene. More than 30 alleles at this locus determine a size polymorphism. The size of the apo(a) isoform is inversely correlated with Lp(a) plasma concentrations, which are non-normally distributed in most populations. To a minor extent, apo(a) gene-independent effects also influence Lp(a) concentrations. These include diet, hormonal status and diseases like renal disease and familial hypercholesterolemia. The standardisation of Lp(a) quantification is still an unresolved problem due to the enormous particle heterogeneity of Lp(a) and homologies of other members of the gene family. Stability problems of Lp(a) as well as statistical pitfalls in studies with small group sizes have created conflicting results. The apo(a)/Lp(a) secretion from hepatocytes is regulated at various levels including postranslationally by apo(a) isoform-dependent prolonged retention in the endoplasmic reticulum. This mechanism can partly explain the inverse correlation between apo(a) size and plasma concentrations. According to numerous investigations, Lp(a) is assembled extracellularly from separately secreted apo(a) and LDL. The sites and mechanisms of Lp(a) removal from plasma are only poorly understood. The human kidney seems to represent a major catabolic organ for Lp(a) uptake. The underlying mechanism is rather unclear; several candidate receptors from the LDL-receptor gene family do not or poorly bind Lp(a) in vitro. Lp(a) plasma levels are elevated over controls in patients with renal diseases like nephrotic syndrome and end-stage renal disease. Following renal transplantation, Lp(a) concentrations decrease to values observed in controls matched for apo(a) type. Controversial data on Lp(a) in diabetes mellitus mainly result from insufficient sample sizes in numerous studies. Large studies and those including apo(a) phenotype analysis have come to the conclusion that Lp(a) levels are not or only moderately elevated in insulin-dependent patients. In non-insulin-dependent diabetics Lp(a) is not elevated. Several rare disorders, such as LCAT and LPL deficiency, as well as liver diseases and abetalipoproteinemia are associated with low plasma levels or lack of Lp(a).

Influence of hematocrit on the measurement of lipoproteins demonstrated by the example of lipoprotein(a)

BACKGROUND

The measurement of many parameters of human blood is usually performed in plasma or serum. Since lipoproteins or apolipoproteins, for example, are found almost exclusively in the plasma fraction after low-speed centrifugation, these parameters can be expected to be distributed in a different plasma volume depending on the hematocrit value. Therefore, the measured plasma levels might be relatively too low or too high in comparison to the whole blood concentrations in the case of abnormal hematocrit levels. The aim of our experiments was to evaluate the extent of differences between whole blood and plasma concentrations, taking as an example lipoprotein(a) [Lp(a)] in hemodialysis patients with documented decreased hematocrit values.

METHODS

Lp(a) was measured in plasma as well as whole blood of 15 hemodialysis patients with low hematocrit values (0.29 +/- 0.02) in comparison to 11 control subjects (0.45 +/- 0.04).

RESULTS

Plasma concentrations were 27% higher in patients than in controls (19.7 vs. 15.5 mg/dl). The relative difference was twice as high (59%) when measured in whole blood (13.5 vs. 8.5 mg/dl). Similar relative differences were observed when whole blood concentrations of 125 hemodialysis patients and 256 controls were calculated with the formula [Lp(a)plasma * (1-hematocrit)].

CONCLUSIONS

Our findings clearly demonstrate that hematocrit is a strong confounding variable of lipoprotein measurement in epidemiological studies when concentrations are measured in plasma, especially in cases of abnormal hematocrit values. Furthermore, studies investigating the longitudinal changes of lipoproteins should consider potential hematocrit changes.

Intracellular metabolism of human apolipoprotein(a) in stably transfected Hep G2 cells

Lipoprotein(a) [Lp(a)] consists of LDL and the glycoprotein apolipoprotein(a) [apo(a)], which are covalently linked via a single disulfide bridge. The formation of Lp(a) occurs extracellularly, but an intracellular assembly in human liver cells has also been claimed. The human apo(a) gene locus is highly polymorphic due to a variable number of tandemly arranged kringle IV repeats. The size of apo(a) isoforms correlates inversely with Lp(a) plasma concentrations, which is believed to reflect different synthesis rates. To examine this association at the cellular level, we analyzed the subcellular localization and fate of apo(a) in stably transfected HepG2 cells. Our results demonstrate that apo(a) is synthesized as a precursor with a lower molecular mass which is processed into the mature, secreted form. The retention times of the precursor in the ER positively correlated with the sizes of apo(a) isoforms. The mature form was observed intracellularly at low levels and only in the Golgi apparatus. No apo(a) was found to be associated with the plasma membrane. Under temperature-blocking conditions, we did not detect any apo(a)/apoB-100 complexes within cells. This finding was confirmed in HepG2 cells transiently expressing KDEL-tagged apo(a). The precursor and the mature forms of apo(a) were found in the ER and Golgi fractions, respectively, also in human liver tissue. From our data, we conclude that in HepG2 cells the apo(a) precursor, dependent on the apo(a) isoform, is retained in the ER for a prolonged period of time, possibly due to an extensive maturation process of this large protein. The assembly of Lp(a) takes place exclusively extracellularly following the separate secretion of apo(a) and apoB.

[Lipoprotein(a)--a risk factor for coronary sclerosis in patients with hypothyroidism]

In a group of patients with developed primary hypothyroidism the authors investigated in a longitudinal eight-month trial the effect of hormonal substitution therapy with thyroxine (T4) on the serum concentration of lipids, apolipoprotein B and lipoprotein (a)--risk factors for the development of early coronary sclerosis. In some patients--"responders"--gradually euhormonosis, normolipaemia are induced and clinical symptoms of hypothyroidism receded. In the second group of patients with hypothyroidism, so-called "non-responders" (n = 5) after eight months substitution treatment with thyroxine the anticipated effect does not occur and the investigated serum parameters improve only partially. The thyroxine, TSH levels and those of lipid parameters and apolipoprotein B persist in the zone of pathological values. The lipoprotein (a) concentration in both groups of patients with hypothyroidism does not change during thyroxine substitution and varies near baseline values. From the submitted observations the authors of the present work do not assume that thyroxine plays a part in the catabolism of lipoprotein (a) via LDL receptors, the activity and number of which increases along with the effect of thyroxine.

Increased plasma concentrations of LDL-unbound apo(a) in patients with end-stage renal disease

Lipoprotein(a) [Lp(a)] and its characteristic glycoprotein apolipoprotein(a) [apo(a)] are risk factors for atherosclerosis in the general population. Patients with renal disease show an elevation of Lp(a). Recent studies have described an arteriovenous difference of Lp(a) in the renovascular bed as well as the plasma-derived fragmented LDL-unbound apo(a) in urine, suggesting that the kidney is involved in the metabolism of Lp(a). We therefore investigated whether patients with chronic renal failure have higher levels of LDL-unbound apo(a) and whether this could account for the increased Lp(a) concentrations in these patients. In addition, we studied the possible generation of apo(a) fragments in vitro by mimicking uremic plasma conditions and by investigating the assembly of Lp(a) in cell culture experiments. Patients treated by hemodialysis (N = 185) and by continuous ambulatory peritoneal dialysis (CAPD; N = 20) had markedly elevated absolute (1.22 +/- 1.55 mg/dl and 2.14 +/- 2.86 mg/dl) as well as relative (7.5% and 7.3%) amounts of LDL-unbound apo(a) in comparison to controls (0.46 +/- 0.48 mg/dl or 4.5%). Following renal transplantation the absolute amount decreased significantly. Lp(a) plasma concentration was the most important determining variable for the absolute amount of LDL-unbound apo(a) and showed a positive correlation in both hemodialysis patients (r = 0.85) and controls (r = 0.92). In vitro experiments demonstrated that "uremization" of plasma samples did not generate a higher amount of LDL-unbound apo(a). Although LDL of renal patients has different chemical and structural properties as compared to control LDL, the extracellular assembly of Lp(a) did not differ between patients and controls. Therefore, the higher amounts of LDL-unbound apo(a) found in renal disease are not caused by an impaired assembly of Lp(a), but rather indicate a catabolic role of the kidney for LDL-unbound apo(a) as was already shown for Lp(a). Despite a small contribution, these elevated levels cannot explain the higher Lp(a) values found in patients with end-stage renal disease.

Biogenesis of lipoprotein(a) in human and animal hepatocytes

The atherogenic plasma lipoprotein complex Lp(a) consists of low density lipoprotein (LDL) and the highly polymorphic glycoprotein apolipoprotein(a) covalently linked by a disulfide bridge. A size polymorphism of apolipoprotein(a) results from a variable number of tandemly arranged kringle IV repeats. The largely varying plasma concentration of Lp(a) is nonnormally distributed in the population and correlates inversely with the molecular mass of apolipoprotein(a). In vivo turnover studies have revealed that differences in Lp(a) plasma concentrations reflect different synthesis rather than degradation. Plasma Lp(a) originates exclusively in the liver. Detailed studies of the intracellular metabolism of apolipoprotein(a) in transfected human hepatoma cells as well as in primary baboon hepatocytes have revealed an unusual secretory pathway of this protein. Due to complex folding and processing, an immature precursor form of apolipoprotein(a) is retained in the endoplasmic reticulum for a prolonged time. This retention leads to a massive accumulation in the endoplasmic reticulum which stands in contrast to most secretory proteins. Since the retention time correlates positively with the apolipoprotein(a) isoform size, this intracellular mechanism could explain the inverse correlation between the isoform size and plasma concentrations observed in the general population. These findings therefore demonstrate a novel cellular regulatory mechanism lor a secretory human plasma protein with genetically controlled concentrations. The majority of the above-mentioned studies revealed another unusual feature of the biogenesis of Lp(a). The mature Lp(a) complex is formed, at least in the investigated cell models, only following separate secretion of apolipoprotein(a) and LDL-like particles. Work that is related to both aspects of Lp(a) formation, both from our laboratory and from other authors, is reviewed.

Apolipoprotein B, fibrinogen, HDL cholesterol, and apolipoprotein(a) phenotypes predict coronary artery disease in hemodialysis patients

Patients with end-stage renal disease have a markedly elevated risk for coronary artery disease (CAD). Lipids and most lipoproteins, however, seem to be not predictive for CAD in these patients. Although there is clear evidence that lipoprotein(a) [Lp(a)] is significantly elevated in these patients, no study with a sufficiently large group of hemodialysis patients has investigated the relationship between CAD and Lp(a), as well as the genetically determined apolipoprotein(a) [apo(a)] phenotype. This cross-sectional study determines the prevalence of CAD in relation to the cardiovascular risk profile in an unselected population of 607 hemodialysis patients, of which 33% were diabetic patients. Twenty-six percent (n = 158) of all patients suffered from CAD as diagnosed by a definitive myocardial infarction (n = 102) and/or at least one stenosis >50% of a coronary artery (n = 143). In univariate analysis, several classic risk factors, including the concentration of lipids, lipoproteins, apolipoproteins, and fibrinogen, correlated with CAD. Lp(a) in patients with CAD showed only a tendency to higher levels, without reaching significance, compared with patients without CAD (26.6 +/- 30.8 mg/dl versus 22.1 +/- 30.4 mg/dl, P = 0.10). The frequency of low molecular weight apo(a) isoforms, however, was significantly greater in the group with CAD (34.8% versus 23.6%, P < 0.01). Stepwise logistic regression analysis found seven variables associated with CAD: apolipoprotein B, the low molecular weight apo(a) phenotype, male sex, age, fibrinogen, diabetes mellitus, and HDL cholesterol. The association of these variables with CAD differed depending on age. These results indicate that, besides classic risk factors such as age, sex, and diabetes mellitus, additional factors of the lipoprotein and fibrinolytic system contribute to the high prevalence of CAD in hemodialysis patients.

LDL-unbound apolipoprotein(a) and carotid atherosclerosis in hemodialysis patients

High lipoprotein(a) [Lp(a)] plasma concentrations, which are genetically determined by apo(a) size polymorphism, are directly associated with an increased risk for atherosclerosis. Patients with end-stage renal disease (ESRD), who show an enormous prevalence of cardiovascular disease, have elevated plasma concentrations of Lp(a). In recent studies we were able to show that apo(a) size polymorphism is a better predictor for carotid atherosclerosis and coronary artery disease in hemodialysis patients than concentrations of Lp(a) and other lipoproteins. Less than 5% of apo(a) in plasma exists in a low-density lipoprotein (LDL)-unbound form. This "free" apo(a) consists mainly of disintegrated apo(a) molecules of different molecular weight, ranging from about 125 to 360 kDa. LDL-unbound apo(a) molecules are elevated in patients with ESRD. The aim of this study was therefore to investigate whether the LDL-unbound form of apo(a) contributes to the prediction of carotid atherosclerosis in a group of 153 hemodialysis patients. The absolute amount of LDL-unbound apo(a) showed a trend to increasing values with the degree of carotid atherosclerosis, but the correlation of Lp(a) plasma concentrations with atherosclerosis was more pronounced. In multivariate analysis the two variables were related to neither the presence nor the degree of atherosclerosis. Instead, the apo(a) phenotype took the place of Lp(a) and LDL-unbound apo(a). After adjustment for other variables, the odds ratio for carotid atherosclerosis in patients with a low molecular weight apo(a) phenotype was about 5 (p<0.01). This indicates a strong association between the apo(a) phenotype and the prevalence of carotid atherosclerosis. Finally, multivariate regression analysis revealed age, angina pectoris and the apo(a) phenotype as the only significant predictors of the degree of atherosclerosis in these patients. In summary, it seems that LDL-unbound apo(a) levels do not contribute to the prediction of carotid atherosclerosis in hemodialysis patients. However, this does not mean that "free", mainly disintegrated, apo(a) has no atherogenic potential.

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.