Genetics of the low density lipoprotein receptor:

Six indices of low density lipoprotein (LDL) receptor activity were assayed in cultured fibroblasts from seven subjects with familial hypercholesterolemia (HC) and six subjects without HC (non-HCs). Four non-HCs, three HC heterozygotes and one HC homozygous proband belonged to one kindred (kindred A). The proband's fibroblast 125I-LDL processing values fell within or were slightly above the range defined by fibroblasts from three "receptor-negative" HC homozygotes. Thus, the plasma membrane receptor defect in this kindred is probably of the "receptor-negative" category. LDL receptor-dependent 125I-LDL processing was about twice as high in fibroblasts from non-HCs as in those from HC heterozygotes belonging to kindred A. The segregation pattern of LDL receptor activity in this kindred was compatible with control by a single gene locus. 125I-LDL processing values from non-HCs, HC heterozygotes and HC homozygotes differed significantly from one another, but non-HCs and HC heterozygotes showed some overlap. LDL receptor-dependent 125I-LDL association (plasma membrane binding plus intracellular accumulation) data for 6 HC heterozygous and 13 non-HC fibroblast strains clustered into two and into three groups, respectively. Median 125I-LDL association levels in these groups appeared to be in agreement with hypothesis that two different geno-types in HC heterozygotes and three in non-HCs determined LDL receptor activity. These findings suggest the possibility that 125I-LDL processing studies may reveal "normal" alleles at the LDL receptor locus.     

Genetics of the low density lipoprotein receptor:

Fibroblast association (plasma membrane binding plus intracellular accumulation) and degradation of radioiodinated low density lipoprotein (125I-LDL) index plasma membrane LDL receptor activity. Cultured fibroblasts from 23 subjects affected with familial hypercholesterolemia (HC) and from 95 subjects without HC (non-HCs) were tested for 125I-LDL association and degradation. Both LDL receptor activity indices were twice as high in non-HC and HC heterozygous cell strains. This is compatible with a major gene effect on LDL receptor activity. However, a considerable overlap between non-HC and HC heterozygous values was found in the 125I-LDL association assay [median (range) 970 (330-2500), and 450 (250-490), respectively] and in the degradation assay [median (range) 810 (280-2020), and 470 (160-790), respectively]. The values are expressed as ng 125I-LDL X mg cell protein-1 X 4.5 h-1. These great overlaps in the LDL receptor activity indices support the view that the influence of LDL receptor activity on the HC phenotype may be smaller than believed previously. Furthermore, for the diagnosis of HC, these LDL receptor activity assays are far more expensive and have less sensitivity and specificity than simple serum cholesterol determination. The LDL receptor-dependent 125I-LDL association values for the HC heterozygous individuals clustered into four groups. Family data supported the hypothesis that this variation could be due to four different LDL receptor variants, each coded for by different alleles at the LDL receptor locus. If confirmed, this finding may have implications for the understanding of the variable expression of HC and also of the genetic impact on lipoprotein metabolism and susceptibility to atherosclerosis in non-HCs.     

Genetics of the low density lipoprotein receptor:

Fibroblast low density lipoprotein (LDL) plasma membrane receptor activity, measured as 125I-LDL association (plasma membrane binding plus intracellular accumulation) and degradation was determined in cell strains from 14 monozygotic (MZ) and 21 like-sexed dizygotic (DZ) normolipidemic twin pairs. The twins were between 57 and 62 years old and had liver apart for an average of 38 years (range 0-60). The intrapair differences were significantly smaller in MZ than in DZ twin pairs in fibroblast 125I-LDL association as well as degradation assays (P less than 0.05). These findings suggest a genetic influence on normal variation in LDL receptor activity in vitro. In two MZ pairs discordant for psoriasis, the psoriatic twin had markedly lower LDL receptor activity than the cotwin.     

Low density lipoprotein receptor activity in cultured skin fibroblasts from octa- and nonagenarians

Low density lipoprotein (LDL) receptor activity determined as association or degradation of 125I-LDL at 37 degrees C, was measured in cultured skin fibroblasts from 48 elderly, independent subjects (84-95 years old). Data concerning these subjects were compared with those from a reference group of 27 younger, healthy subjects (5-64 years old) and 23 heterozygotes for familial hypercholesterolemia (9-67 years old). The median (10th-90th percentile) association and degradation value for the older subjects were 294 (172-535) ng 125I-LDL/mg protein/6h and 191 (99-376) ng 125I-LDL/mg protein/6h, respectively. These values did not differ significantly from the corresponding values in the normal subjects (296 (60-491) ng 125I-LDL/mg protein/6h and 171 (99-275) ng 125I-LDL/mg protein/6h, respectively). Thus, maximal LDL receptor activity in fibroblasts from the older subjects seems to be comparable to that of younger, healthy controls. However, the elderly subjects had markedly higher values for total serum cholesterol and LDL cholesterol than the normal controls. There was a significant increase in LDL receptor activity with age in the normal controls. Such an increase was not found in heterozygotes for familial hypercholesterolemia.     

Low density lipoprotein receptor activity in cultured fibroblasts from subjects with or without ischemic heart disease (in the absence of familial hypercholesterolemia)

Fibroblast strains from six subjects with ischemic heart disease (IHDs) were compared to strains from 43 subjects without a history of IHD (non-IHDs), with respect to association (plasma membrane binding plus intracellular accumulation) and degradation of radioiodinated LDL (125I-LDL). The subjects (25 females and 24 males) were selected on the criteria that they were twins (one from each pair), 58-61 years old, and living within 200 km of Oslo. None of them suffered from autosomal, dominant hypercholesterolemia, which is associated with reduced cell surface LDL receptor activity and increased susceptibility to IHD. There was a trend towards lower 125I-LDL association values in strains from IHDs than in strains from non-IHDs (P=0.009). There was a significant negative correlation between, on one hand, serum total cholesterol level and on the other fibroblast association (P=0.03) or degradation (P=0.04) of 125I-LDL. We have previously presented data indicating that fibroblast association of LDL may be determined by alternate genes at one single locus. Together with the present limited data, this raises the possibility that normal genes at the LDL receptor locus may render subjects more or less susceptible to ischemic heart disease.     

Oxidative modifications of LDL increase its binding to extracellular matrix from human aortic intima: influence of lesion development, lipoprotein lipase and calcium

Retention of atherogenic lipoproteins in the arterial intima by extracellular matrix (ECM) is assumed to occur during early atherogenesis and its further development. Low density lipoprotein (LDL) trapped in the intima may undergo oxidative modifications, which initiate a chain reaction in atherogenesis. Lipoprotein lipase (LPL) has been found to mediate the binding of native and oxidized LDL to ECM produced by cultured cells and to contribute to foam cell formation by mildly oxidized LDL. In this study ECM, isolated from human aortic intima with different atherosclerotic lesions, was used for the first time to measure the binding to it in vitro of native and differently oxidized 125I-LDL. Oxidation of 125I-LDL increased its binding to the ECM, which was most prominent with the material isolated from intima at the early stage of atherogenesis. With the progression of atherosclerosis, the ability of the isolated intimal ECM to bind native and oxidized 125I-LDL decreased, and strongly oxidized 125I-LDL decreased more than native and moderately oxidized 125I-LDL. LPL increased the binding of moderately oxidized 125I-LDL to the ECM more than native 125I-LDL, while it had only a small effect on strongly oxidized 125I-LDL. LPL-mediated binding of native and oxidized 125I-LDL decreased with the development of atherosclerotic lesions. Calcium ions also increased the binding of LDL to the ECM. This enhanced binding increased with the extent of LDL oxidation, especially at the early stage of atherogenesis, and decreased with lesion progression. These data suggest that the ability of ECM to retain LDL in arterial intima depends on LDL oxidation status and changes with the progression of atherogenesis. In addition, LPL and calcium ions may participate in the retention of LDL in vivo.     

离体培养动脉内皮细胞结合、内移和降解脂蛋白(α)作用的研究

本文研究离体培养动脉内皮细胞通过LDL受体结合、内移和降解脂蛋白(α)[lipopro-tein(a),Lp(α)]的作用.实验取第3～4代离体培养的动脉内皮细胞分为两组,对照组培养液中不加碘标的脂蛋白(a)[~(125)I—Lp(α)],实验组培养液中加~(125)I—Lp(α)最终浓度分别为0.25、0.5、1、10和50μg/ml.用γ计数器测定内皮细胞对Lp(α)的结合、内移和降解值,并测定内皮细胞的蛋白质量,以μg/mg细胞蛋白表示.结果随培养液中加进~(125)I—Lp(α)的浓度递增而增加,在10μg/ml浓度时最大的结合、内移和降解值分别是0.219、0.390和0.465μg/mg细胞蛋白;最大的结合率、内移率和降解率分别是2.19%、3.90%和4.65%.结果提示Lp(α)通过动脉内皮细胞LDL受体途径的摄取和降解作用是非特异性的.     

脂蛋白（a）及低密度脂蛋白与成纤维细胞表面结合特性的比较

脂蛋白（a)[Lipoprotein(a),Lp(a)]的代谢途径尚有争议。本文比较研究了Lp(a)和低密度脂蛋白（Low Density Lipoprotein,LDL)与成纤维细胞表面的结合特性，结果显示，Lp(a)与成纤维细胞呈特异性，高亲和力，可饱和性结合，但是Lp(a)与细胞的亲和力，最大结合容量均比LDL低，竞争性结合试验显示LDL只能部分抑制Lp(a)与成纤维细胞表面特异性的结合；细胞表面LDL受体活性的下调，可使LDL与细胞的结合量下降58％，而Lp(a)的结合量仅下降12．3％，这些结果表明LDL受体只能参与部分Lp(a)与细胞表面的结合及代谢。     

Low density lipoprotein receptors in cultured skin fibroblasts from psoriasis patients

Low density lipoprotein (LDL) receptor activity, measured as 125I-LDL association and degradation at 37 degrees C, was determined in cultured fibroblasts from involved as well as uninvolved skin obtained from 20 psoriasis patients. The same analyses were conducted in fibroblasts from two reference groups consisting of 19 heterozygotes for familial hypercholesterolemia and 16 normal subjects, respectively. Psoriasis patients had significantly lower LDL receptor activity than normals, and it was comparable to that of the heterozygotes for familial hypercholesterolemia. The reduced LDL receptor activity was not accompanied by an increase in total serum cholesterol. The psoriasis patients had a significant reduction in apo-B concentration, but did not differ from the normals in the other serum lipid or lipoprotein parameters. There was no difference in LDL receptor activity between involved and uninvolved skin from psoriasis patients. These results suggest that there is an abnormal cell membrane in dermal fibroblasts from psoriasis patients. Since their total serum cholesterol is normal, their low LDL receptor activity may be confined to dermal cells, leaving the hepatic lipid metabolism normal. The pathogenetic significance of this finding is unknown.     

Lipoprotein (a), low-density,intermediate-density lipoprotein,and blood pressure in a young male population

Summary Both hypercholesterolemia and hypertension are risk factors for atherosclerotic vascular disease, and elevated cholesterol levels occur more frequently than expected in patients with hypertension. Elevated levels of intermediate-density lipoproteins (IDL) and low-density lipoproteins (LDL) were shown to be atherogenic, and LDL, comprising the major cholesterol-carrying fraction in human plasma, are structurally related to lipoprotein (a) [Lp(a)], a further risk factor for atherosclerosis. In the present study we investigated 200 male employees (mean age 26±7 years) to determine whether the relationship of IDL and Lp(a) to systemic blood pressure is similar to the reported correlations between total and LDL cholesterol and systemic blood pressure. To this end blood pressure was measured several times in each individual, and lipids, lipoprotein-cholesterol, apolipoprotein B (apo B), and Lp(a) were determined in fasting serum. IDL cholesterol and apo B, the main protein component of IDL and LDL correlated with blood pressure. However, levels of Lp(a) correlated neither with systolic or diastolic blood pressure nor with lipoprotein cholesterol, body weight, or age. Although IDL and Lp(a) are considered lipoprotein risk factors for atherosclerosis, levels of Lp(a), unlike IDL, are not related to blood pressure, body weight, or age. Our data suggest different metabolic and pathophysiological mechanisms of the risk factors, IDL, LDL, and Lp(a).Abbreviations VLDL very low density lipoprotein - IDL intermediate-density lipoprotein - LDL low-density lipoprotein - ApoB Apolipoprotein B - Lp(a) lipoprotein (a) - BMI body mass index Dedicated to Prof. Dr. N. Zöllner on the occasion of his 70th birthday     

Low-density lipoprotein endocytosis. I. Influence of the multivalent ligand cationized ferritin on normal and receptor-negative human fibroblasts

Based upon the observation that the multivalent ligand cationized ferritin (CF) alters the cell surface distribution of anionic domains and significantly enhances the adsorptive endocytosis of 125I-labeled human serum albumin, these studies were undertaken to probe the influence of CF on receptor-mediated low-density lipoprotein (LDL) endocytosis and the nature of the mechanisms involved. A brief 1-min exposure of normal receptor upregulated fibroblasts to CF (0.2 mg/ml) resulted in a significant decrease (P less than 0.001) in the subsequent internalization and degradation of 125I-LDL. Studies with receptor downregulated normal fibroblasts indicated that CF pretreatment did not measurably influence 125I-LDL internalization and only slightly inhibited its degradation (P less than 0.05). In contrast, CF pretreatment of FH receptor-negative mutant skin fibroblasts resulted in a modest but significant increase in both 125I-LDL internalization and degradation (P less than 0.05). Scatchard analyses of binding data indicated that CF-pretreated upregulated normal fibroblasts exhibit a single class of LDL binding sites with an affinity, Kd = 24.7 +/- 4.1 nM, almost 10-fold lower than the affinity of binding sites in untreated controls, Kd = 3.2 +/- 0.06 nM. Increasing either the concentration or the duration of CF exposure resulted in additional inhibition of LDL internalization and degradation associated primarily with a decrease in the number of LDL binding sites without any further change in binding affinity. Total cellular LDL receptor-mediated binding, measured using an octylglucoside solubilization-filtration assay, confirmed the CF-induced decrease in high-affinity LDL binding. Pulse-chase experiments showed that CF had no direct influence on LDL degradation, nor did it influence targeting of the LDL-containing endosome toward exocytosis. Further, restoration of LDL receptor function to control values after CF pretreatment required de novo protein synthesis. The normal feedback inhibition of HMG-CoA reductase activity was nearly abolished by CF pretreatment. Additionally, CF pretreatment was found to induce not only a redistribution of surface anionic sites, but also a very rapid internalization of surface components labeled with 4,4'-[3H]diisothiocyano-1,2..diphenylethane-2,2'-disulfonic acid. It is concluded that the inhibitory influence of CF on LDL endocytosis is mediated via a decrease in the affinity and in the number of functional LDL receptors.     

Maximal low density lipoprotein receptor activity and the effect of lipid lowering diet on total serum cholesterol

This study was undertaken to test if the effect of lipid lowering diet on total serum cholesterol, is influenced by maximal low density lipoprotein (LDL) receptor activity. LDL receptor activity was determined in cultured skin fibroblasts from hypercholesterolemic, male subjects after lipid lowering diet intervention. The LDL receptor values from 15 subjects (responders) who had responded well to a lipid lowering diet and from 14 subjects (non-responders) who had responded poorly, were compared. The responders had a reduction in total serum cholesterol of 29.4%, and the non-responders had a reduction of 8.2% (p less than 0.0001). The higher values for LDL receptor activity among the responders did not reach statistical significance. For all 29 subjects there were non-significant positive correlations between reductions in total serum cholesterol and values for association or degradation of 125I-LDL at 37 degrees C (r = 0.16, p = 0.40 and r = 0.17, p = 0.38, respectively). Thus, it seems that maximal LDL receptor activity is not a major predictor for the response of lipid lowering diet on total serum cholesterol levels in hypercholesterolemic subjects without autosomal dominant familial hypercholesterolemia.     

Elevated lipoprotein(a) levels in patients with acute myeloblastic leukaemia decrease after successful chemotherapeutic treatment

Summary Twenty-two patients with acute myeloblastic leukaemia (AML) were studied to investigate disease-associated changes in lipid metabolism. Lipoprotein (a) [Lp(a)] levels were found to be elevated at the time of diagnosis (median 23 mg/dl; 41% of patient group had levels greater than 25 mg/dl) and diminished after successful chemotherapeutic treatment in 9 of 10 cases, with a maximum decrease from 56 to 10 mg/dl. In contrast, reduced levels of total cholesterol, low density lipoprotein (LDL) and high density lipoprotein (HDL) (medians 137, 87 and 20 mg/dl, respectively) were observed at the time of diagnosis. Cholesterol and HDL levels increased in all 10 and LDL in 9 cases in which complete remission was achieved. These data suggest that the catabolism of LDL-cholesterol might be even more enhanced than assumed to date. Furthermore, it indicates that the Lp(a) level in acute myeloblastic leukaemia is influenced either directly or indirectly by the leukaemic blasts.Abbreviations AML acute myeloblastic leukaemia - Lp(a) lipoprotein (a) - LDL low density lipoprotein - HDL high density lipoprotein - FAB French-American-British - CALGB cancer and leukaemia group B - CR complete remission - TG triglycerides - VLDL very low density lipoprotein - RIA radioimmunoassay - apo(a) apoprotein (a) - HMG-CoA reductase 3-hydroxy-3-methylglutaryl coenzyme A reductase Supported by the Volkswagen Stiftung with a grant to A.N.     

In vitro studies of the interaction of calcium ions and other divalent cations with the Lp(a) lipoprotein and other isolated serum lipoproteins

The interaction of isolated Lp(a) lipoprotein with different divalent cations was studied and compared to that of other isolated lipoprotein classes.
Purified Lp(a) lipoprotein was found to be most sensitive to the metal ions tested, and the Lp(a) lipoprotein was the only lipoprotein which was precipitated by calcium ions alone. The precipitation apparently depends on the ionic radii of the cations used as well as on the lipoprotein class tested. The precipitation reaction between calcium ions and the Lp(a) lipoprotein, and the interaction between calcium ions and LDL (without precipitation) seem to follow the known rules for small ion - macromolecule interaction reasonably well. The calcium ion - Lp(a) lipoprotein interaction results in a small aggregate. The binding is of ionic type and the precipitation reaction is initially reversible. It was estimated that LDL particles have a mean of 290 equivalent and non-interacting binding sites for calcium ions.
The above observations concerning the Lp(a) lipoprotein may be of interest in view of the significantly higher frequency of early coronary heart disease in Lp(a+) than in Lp(a-) individuals, and in view of the previously reported biochemical differences between individuals of different Lp phenotype.     

Effect of low-density lipoprotein buoyant density and cholesterol content on the formation of lipoprotein(a) particles

Lipoprotein(a) [Lp(a)] is a unique lipoprotein which resembles low-density lipoprotein (LDL) both in lipid composition and the presence of apolipoprotein B-100 (apo B-100). Lp(a) is, however, distinguishable from LDL by the presence of an additional glycoprotein apolipoprotein(a) [apo(a)], which is covalently attached to apo B-100 by a single disulfide bond. It is now generally accepted that Lp(a) assembly is a two-step process in which the initial non covalent interaction between apo(a) and apo B-100 is mediated by the weak lysine binding sites present in kringle IV types 6, 7 and 8 of apo(a). In the present study, we have investigated the effect of LDL heterogeneity on Lp(a) assembly in a group of 111 individuals. The three parameters of LDL composition assessed in this study were the cholesterol content, the apo B content, and the relative flotation rate (a measure of LDL buoyancy and thus size). We found no correlation between the size of LDL particles and the extent of Lp(a) formation; a weak negative correlation was observed between cholesterol content of LDL and Lp(a) formation (P=0.042). This may suggest a role for free (i. e., surface-associated) cholesterol in the ability of LDL to form Lp(a) particles. Received: 1 June 2001 / Accepted: 2 July 2001     

Receptor-mediated low-density lipoprotein catabolism

Summary Low-density lipoprotein (LDL) receptors are demonstrable in cultured fibroblasts from normal subjects but are decreased or absent in cells from patients with heterozygous or homozygous familial hypercholesterolaemia. In vivo receptor-mediated LDL catabolism, determined as the difference between the turnover rates of¹²⁵I-LDL and¹³¹I-LDL coupled with cyclohexanedione, is responsible for approximately one-third of the total catabolism of LDL in normal subjects, but less than one-fifth in heterozygotes and is totally absent in homozygotes. Receptor-mediated catabolism can be stimulated in normal subjects and in heterozygotes by measures that promote bile acid synthesis, namely, administration of anion-exchange resins or creating a partial ileal bypass. Studies in dogs have shown that such measures stimulate the high-affinity binding of LDL by liver cell mebranes. Taken together, these observations suggest the existence of LDL receptors in human liver, the function of which is to maintain cholesterol homeostasis within the hepatocyte during periods of increased demand. Partial or complete absence of such hepatic receptors may play a major role in the pathogenesis of familial hypercholesterolaemia.     

内源性高甘油三酯血症患者血浆VLDL、LDL及HDL对血凝的影响

目的：研究内源性高甘油三酯血症(HTG)患血浆极低密度脂蛋白(VLDL)、低密度脂蛋白(LDL)及高密度脂蛋白(HDL)是否发生了氧化修饰及其对血凝的影响。方法：对2l例内源性高甘油三酯血症患与2l例年龄性别相近的正常人的血脂、脂质过氧化物进行了分析。用一次性密度梯度超速离心法分离血浆VLDL、LDL及HDL，测定这三种脂蛋白的234nm光吸收、相对电泳迁移率(REM)和硫代巴比妥酸反应物质(TBARS)，分别将这三种脂蛋白加入由正常人新鲜混合血浆构成的反应系统中，按试剂盒分别测定凝血酶原时间(PT)及活化部分凝血酶原时间(APIT)。结果：内源性HTG患血浆TG含量平均升高2．73倍，HDLC下降l.7l倍，同时LPO升高1．22倍;HTG组VLDL、LDL及HDL的REM、234nm光吸收值、TBARS含量均较对照组显增加(P＜0．01)，表明内源性HTG患血浆VLDL、LDL及LDL均发生了氧化修饰生成Ox—VLDL、Ox-LDL.PT及APTT在分别加入HTG组的VLDL、LDL及HDL后均比加入相应正常组脂蛋白明显缩短(P均＜0．05)。相关分析表明，HTG组血浆VLDL及HDL相对电泳迁移率(REM)与PT呈负相关(P＜0．01)。结论：HTG患血浆VLDL、LDL及HDL发生了氧化修饰，并使PT及APTT明显缩短。     

Measurement of 125I-low density lipoprotein uptake in selected tissues of the squirrel monkey by quantitative autoradiography.

A recently developed technique of absolute quantitative light microscopic autoradiography of 125I-labeled proteins in biologic specimens was used to measure 125I-low density lipoprotein (125I-LDL) concentration levels in various tissues of the squirrel monkey after 30 minutes of in vivo LDL circulation. Liver and adrenal cortex exhibited high 125I-LDL concentrations, presumably because of binding to specific cell surface receptors and/or internalization in vascular beds with high permeability to LDL. High tissue concentrations of LDL were associated with the zona fasciculata and reticularis of the adrenal cortex and the interstitial cells of Leydig in the testis; significantly lower levels of 125I-LDL were observed in the adrenal medulla, the zona glomerulosa, and germinal centers of the testis. Contrary to previous reports, low 125I-LDL concentrations were observed throughout the gastrointestinal tract and in lymph nodes. In addition, multiple arterial intramural focal areas of high 125I-LDL concentrations were identified in arteries supplying the adrenal gland, lymph node, small bowel, and liver.     

Apolipoprotein(a) phenotypes and lipoprotein(a) concentrations in patients with hyperthyroidism

Lipoprotein(a) [Lp(a)] is a low-density lipoprotein (LDL) particle in which apolipoprotein B-100 (apoB) is attached to a glycoprotein called apolipoprotein(a) [apo(a)]. Apo(a) has several genetically determined phenotypes differing in molecular weight, to which Lp(a) concentrations in plasma are inversely correlated. High plasma levels of Lp(a) are associated with atherosclerotic diseases. It is therefore of interest to study whether factors other than the apo(a) gene locus are involved in the regulation of Lp(a) concentrations. We measured plasma concentrations of Lp(a) and other lipoproteins and determined apo(a) phenotypes in 31 patients with hyperthyroidism, before and after the patients had become euthyroid by treatment. The mean concentration of LDL cholesterol rose from 2.67 to 3.88 mmol/l (P<0.01), apoB rose from 0.79 to 1.03 g/l (P<0.01), and the median Lp(a) concentration increased from 9.74 to 18.97 mg/dl (P<0.01) on treatment. Lp(a) concentrations were inversely associated to the size of the apo(a) molecule both before (P< 0.01) and after treatment (P<0.01). The increase in Lp(a) was significant patients with high molecular weight apo(a) phenotypes (n = 9; P<0.01) and in patients with low molecular weight apo(a) phenotypes (n=16; P< 0.01), but not in those with apo(a) null types (n = 6; P = 0.5). The low levels LDL cholesterol and apoB in untreated hyperthyroidism may result from increased LDL receptor activity. The increase in Lp(a) levels were not correlated with the increase in LDL cholesterol or apoB. Most other clinical evidence indicates that the LDL receptor is not important in Lp(a) catabolism, and we suggest that the low Lp(a) levels seen in thyroid hormone excess are caused by an inhibition of Lp(a) synthesis.Abbreviations Lp(a) lipoprotein(a) - apo(a) apolipoprotein(a) - apoB apolipoprotein B-100 - LDL low-density lipoprotein - HDL high-density lipoprotein - TG triglycerides - T ₄ thyroxine - T ₃ triiodothyronine - TSH thyrotropin     

Low density lipoprotein receptor expression and function in human polymorphonuclear leucocytes

Low density lipoprotein receptors (LDLR), capable of internalizing LDL, are expressed in polymorphonuclear neutrophils (PMN). The expression was assessed using anti-LDLR antibody by flow cytometry. The internalization of LDL was assessed by: (i) quantification of the uptake of labelled LDL with 1,1′-dioctadecyl-3,3,3′,3′ tetramethyl-indocarboxycyanine perchlorate (DiI) by flow cytometry; and (ii) the binding of LDL-¹²⁵I. In fresh purified cells, Lineweaver–Burk analysis of LDL binding (LDL-DiI) revealed that the calculated Kd (internalized LDL) for PMN (15.0 × 10⁻⁹ m) is lower than the Kd for monocytes (1.1 × 10⁻⁷ m) and the Kd for lymphocytes (3.2 × 10⁻⁷ m). Scatchard analysis (LDL-¹²⁵I) revealed 25 000 binding sites and a Kd of 9.6 × 10⁻⁹ m for PMN. The interaction of LDL with its receptor caused a two-fold fast (peak at 1 min) and transient increase in the oxidative burst, measured by the formation of 2′,7′ dicholoflurescein (DCF) by flow cytometry. This effect was not observed in monocytes or lymphocytes, and it was blocked by anti-LDLR antibody. The stimulation of LDL was optimal at 10 μg of protein/ml. LDL was able to suppress DCF formation induced by phorbol myristate acetate (PMA) and PMA was unable to further stimulate LDL-treated cells, suggesting protein kinase-C (PKC) involvement in LDL effects. Using a PKC assay, LDL was shown to induce a two-fold increase in PKC translocation to the membrane. Thus, LDL increases PMN oxidative burst through a PKC-dependent pathway.