Lipoprotein heterogeneity and apolipoprotein B metabolism

The apolipoprotein B containing lipoproteins VLDL, IDL, and LDL exhibit variation in their structure, function, and metabolism. These major lipoprotein classes can be fractionated into apparently discrete components by density gradient centrifugation or affinity chromatography. Examination of the behavior of subfractions in vivo reveals the presence of metabolic channels within the VLDL-LDL delipidation cascade so that the pedigree of a lipoprotein in part determines its metabolic fate. Evidence from VLDL and LDL apoB turnovers together with epidemiological data allows the construction of a quantitative model for the generation of small, dense LDL. This lipoprotein subspecies is one component of the dyslipidemic syndrome known as the atherogenic lipoprotein phenotype, a common disorder in those at risk for coronary heart disease. Understanding lipoprotein heterogeneity is an essential step in the further discovery of the pathogenesis of atherosclerosis and in the tailoring of pharmacologic treatment for subjects at risk.     

An MTP inhibitor that normalizes atherogenic lipoprotein levels in WHHL rabbits

Patients with abetalipoproteinemia, a disease caused by defects in the microsomal triglyceride transfer protein (MTP), do not produce apolipoprotein B-containing lipoproteins. It was hypothesized that small molecule inhibitors of MTP would prevent the assembly and secretion of these atherogenic lipoproteins. To test this hypothesis, two compounds identified in a high-throughput screen for MTP inhibitors were used to direct the synthesis of a highly potent MTP inhibitor. This molecule (compound 9) inhibited the production of lipoprotein particles in rodent models and normalized plasma lipoprotein levels in Watanabe-heritable hyperlipidemic (WHHL) rabbits, which are a model for human homozygous familial hypercholesterolemia. These results suggest that compound 9, or derivatives thereof, has potential applications for the therapeutic lowering of atherogenic lipoprotein levels in humans.     

The compositional abnormalities of lipoproteins in diabetic renal failure

BACKGROUND: Diabetic nephropathy (DN) is a common cause of chronic renal failure (CRF). Patients with DN have abnormal lipoprotein metabolism that can be influenced by both the impairment of renal function and the metabolic control of diabetes. The aim of the study was to explore the specific compositional lipoprotein abnormalities in patients with insulin-dependent DN in comparison with diabetic patients without nephropathy and non-diabetic CRF patients. METHODS: The lipid and apolipoprotein (apo) composition of major lipoprotein density classes was determined in 20 patients with insulin-dependent diabetes mellitus and nephropathy and compared with that in seven diabetic patients without nephropathy, 20 patients with non-diabetic CRF, and nine healthy control subjects. Lipoproteins isolated by preparative ultracentrifugation were very-low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). RESULTS: Patients with DN had a plasma lipid and apolipoprotein profile characteristic of renal dyslipoproteinaemia with increased concentrations of triglycerides and cholesterol, reduced levels of apoA-I and apoA-II and increased levels of apoB, apoC-II, apoC-III and apoE. These changes were more pronounced in diabetic than in non-diabetic patients with comparable degrees of renal failure. All density classes were characterized by abnormal concentration and composition of some lipid and apolipoprotein constituents. DN patients had a more than four-fold increase of VLDL mass, a three-fold increase of IDL mass, and a significant reduction of HDL mass compared to control subjects. They also had significantly higher concentrations of apoB, apoC-peptides and apoE particularly in VLDL and IDL, and to some extent in LDL. In HDL, DN patients had lower cholesterol, apoA-I, apoA-II and apoC-II levels than controls. The major compositional change in DN patients was a significant increase in the relative content of apoC-peptides in IDL and LDL. The lipoprotein abnormalities were more pronounced in patients with high HbA1c values. In addition, lower GFR and increased proteinuria were associated with higher concentrations of triglycerides and apoC peptides in VLDL, IDL, and LDL in DN patients. CONCLUSIONS: The results indicate that patients with DN share the characteristic features of dyslipidaemia of CRF with accumulation of intact or partially delipidized apoB-containing lipoproteins enriched in apoC-peptides and apoE, which are present not only in VLDL and IDL but also in LDL density range. The alterations are more marked in DN than in nondiabetic CRF patients reflecting the additional impact of metabolic control. Increased levels of these lipoproteins may represent risk factors for the accelerated development of atherosclerotic vascular disease in these patients.     

Genetics of type III hyperlipoproteinemia

One hundred forty-seven relatives of 43 patients with "classical" type III hyperlipoproteinemia (HLP) having the apolipoprotein (apo) E2/2 phenotype were studied to determine the occurrence of hyperlipidemia and the presence of further possible genes for lipoprotein disorders in these families. In 12 pedigrees primary dyslipidemia was prevalent among patients and respective blood-relatives. In these kindreds the coexistent presence of genes for familial combined hyperlipidemia (n = 6), familial hypertriglyceridemia (n = 5), and familial hypercholesterolemia (n = 1), respectively, was supposed. Our results, therefore, confirm and extend previous data on the multifactorial genesis of the diseases. Besides homozygosity for a receptor binding-defective isoform of apo E (apo E2), additional genes for familial lipoprotein disorders might operate in the pathogenesis of type III HLP. This is the largest family study performed so far in this primary lipoprotein disorder.     

A gene-targeted mouse model for familial hypobetalipoproteinemia. Low levels of apolipoprotein B mRNA in association with a nonsense mutation in exon 26 of the apolipoprotein B gene

Familial hypobetalipoproteinemia, a syndrome characterized by abnormally low plasma levels of low density lipoprotein cholesterol, is caused by mutations in the apolipoprotein (apo) B gene that interfere with the synthesis of a full-length apoB100. In many cases of familial hypobetalipoproteinemia, nonsense or frameshift mutations result in the synthesis of a truncated apoB protein. To understand why these mutations result in low plasma cholesterol levels, we used gene targeting in mouse embryonic stem cells to introduce a nonsense mutation (N1785Stop) into exon 26 of the mouse Apob gene. The sole product of this mutant Apob allele was a truncated apoB, apoB39. Mice homozygous for this "apoB39-only" (Apob39) allele had low plasma levels of apoB39 and markedly reduced plasma levels of very low density lipoprotein and low density lipoprotein cholesterol when fed a high fat diet. Analysis of liver and intestinal RNA from heterozygous apoB39-only mice revealed that the Apob39 mRNA levels were 60-70% lower than those from the wild-type allele. Interestingly, apoB39 was not cleared as rapidly from the plasma as apoB48. The apoB39-only mice provide new insights into the mechanisms of familial hypobetalipoproteinemia and the structural features of apoB that are important for lipoprotein metabolism.     

Mice overexpressing human lecithin: cholesterol acyltransferase are not protected against diet-induced atherosclerosis

Lecithin: cholesterol acyltransferase (LCAT) (EC 2.3.1.43) is generally assumed to participate in reverse cholesterol transport, i.e., cholesterol transport from peripheral tissues to the liver. LCAT is secreted by the liver and transported in plasma mostly associated with high density lipoprotein. It catalyzes the esterification of cholesterol, mainly high density lipoprotein cholesterol, and produces cholesteryl ester and lysolecithin. Transgenic mice overexpression human LCAT on a C57BL/6 background have elevated high density lipoprotein cholesterol and markedly reduced low and very low density lipoprotein cholesterol and triglyceride levels in plasma, suggesting that such mice may be less susceptible to diet-induced atherosclerosis than isogenic nontransgenic controls. To determine if the apparent anti-atherogenic lipoprotein profile of the LCAT transgenics reduced their susceptibility to atherogenesis, the atherosclerotic lesions developing in transgenic LCAT mice and controls when fed an atherogenic diet were compared by histology and morphometry. Histological examination of the aortas from mice fed a high fat diet for 12, 17 and 22 weeks revealed that the aortic lesions were no smaller or less developed in the transgenic LCAT mice than in the C57BL/6 controls. After 17 weeks there were significantly more "fatty streaks" in the transgenic mice than in the controls. Thus, overexpression of human LCAT in transgenic mice, in spite of their very favourable blood lipoprotein and lipid profile, does not protect against development of atherosclerosis.     

Dramatically decreased high density lipoprotein cholesterol, increased remnant clearance, and insulin hypersensitivity in apolipoprotein A-II knockout mice suggest a complex role for apolipoprotein A-II in atherosclerosis susceptibility

Apolipoprotein (apo) A-II is the second most abundant apolipoprotein in high density lipoprotein (HDL). To study its role in lipoprotein metabolism and atherosclerosis susceptibility, apo A-II knockout mice were created. Homozygous knockout mice had 67% and 52% reductions in HDL cholesterol levels in the fasted and fed states, respectively, and HDL particle size was reduced. Metabolic turnover studies revealed the HDL decrease to be due to both decreased HDL cholesterol ester and apo A-I transport rate and increased HDL cholesterol ester and apo A-I fractional catabolic rate. The apo A-II deficiency trait was bred onto the atherosclerosis-prone apo E-deficient background, which resulted in a surprising 66% decrease in cholesterol levels due primarily to decreased atherogenic lipoprotein remnant particles. Metabolic turnover studies indicated increased remnant clearance in the absence of apo A-II. Finally, apo A-II deficiency was associated with lower free fatty acid, glucose, and insulin levels, suggesting an insulin hypersensitivity state. In summary, apo A-II plays a complex role in lipoprotein metabolism, with some antiatherogenic properties such as the maintenance of a stable HDL pool, and other proatherogenic properties such as decreasing clearance of atherogenic lipoprotein remnants and promotion of insulin resistance.     

Characterization of six patients who are double heterozygotes for familial hypercholesterolemia and familial defective apo B-100

Familial defective apolipoprotein B-100 (FDB) and familial hypercholesterolemia (FH) are the common causes of monogenic primary hypercholesterolemia. An individual of mixed English and Afrikaner descent with both FDB and the FH Afrikaner-1 low-density lipoprotein receptor mutation was identified in our laboratory. Subsequent analysis of her extended family revealed the presence of heterozygotes for either FH Afrikaner-1, FH Afrikaner-2, or FDB as well as five additional double heterozygotes for FH Afrikaner-1 and FDB and one "complex" heterozygote with all three mutations. The hypercholesterolemic and clinical features of the pure FDB subjects were similar to those of the pure FH heterozygotes. The double heterozygotes with both FH and FDB have lipid levels and clinical features that are intermediate in severity between heterozygous and homozygous FH.     

Protection against atherogenesis in mice mediated by human apolipoprotein A-IV

Apolipoproteins are protein constituents of plasma lipid transport particles. Human apolipoprotein A-IV (apoA-IV) was expressed in the liver of C57BL/6 mice and mice deficient in apoE, both of which are prone to atherosclerosis, to investigate whether apoA-IV protects against this disease. In transgenic C57BL/6 mice on an atherogenic diet, the serum concentration of high density lipoprotein (HDL) cholesterol increased by 35 percent, whereas the concentration of endogenous apoA-I decreased by 29 percent, relative to those in transgenic mice on a normal diet. Expression of human apoA-IV in apoE-deficient mice on a normal diet resulted in an even more severe atherogenic lipoprotein profile, without affecting the concentration of HDL cholesterol, than that in nontransgenic apoE-deficient mice. However, transgenic mice of both backgrounds showed a substantial reduction in the size of atherosclerotic lesions. Thus, apoA-IV appears to protect against atherosclerosis by a mechanism that does not involve an increase in HDL cholesterol concentration.     

Lack of association of the apolipoprotein A-I-C-III-A-IV gene XmnI and SstI polymorphisms and of the lipoprotein lipase gene mutations in familial combined hyperlipoproteinemia in French Canadian subjects

Familial combined hyperlipoproteinemia (FCH) is a common familial lipoprotein disorder characterized by elevated plasma cholesterol and triglyceride levels with segregation in first-degree relatives. Most affected subjects with FCH have elevated plasma levels of apolipoprotein (apo) B. The disorder results from oversecretion of hepatic apoB-containing lipoprotein particles. The genetic defect(s) are unknown. Previous work has suggested that genetic polymorphisms of the apoA-I gene and functional abnormalities of the lipoprotein lipase (LPL) gene are associated with FCH. We investigated the XmnI and SstI restriction fragment length polymorphisms (RFLP) of the apoA-I gene in FCH subjects of French Canadian descent. We also investigated three common functional mutations of the lipoprotein lipase (LPL) gene (LPLGly188Glu, LPLPro207Leu, and LPLAsp250Asn) in French Canadians that account for approximately 97% of cases of complete LPL deficiency in the province of Québec, Canada. We identified and characterized 54 FCH probands in lipid clinics and examined at least one first-degree relative. There were 37 men and 17 women (mean age 48 +/- 9 and 58 +/- 8 years, respectively). None of the probands had diabetes mellitus; mean plasma glucose was 5.5 mmol/L. High blood pressure was diagnosed in 32% of men and 29% of women. The body mass index (weight (kg)/height(m2)) was elevated in probands (27 +/- 4 for men and 26 +/- 4 for women). Mean plasma levels of cholesterol (C) was 7.6 +/- 1.5 mmol/L, triglycerides 3.5 +/- 1.6 mmol/L, LDL-C 4.9 +/- 1.2 mmol/L, HDL-C 1.0 +/- 0.3 mmol/L, and apoB 1.83 +/- 0.67 g/L in the probands. Allele frequency of the rare alleles of the XmnI and SstI RFLP was not significantly different from a healthy reference group. In several families studied, the XmnI and SstI RFLP did not unequivocally segregate with the FCH phenotype. There was no significant effect of the presence or absence of the XmnI or SstI RFLP's on plasma lipids, lipoprotein cholesterol or apoB levels. Only one FCH proband was found to have a mutation of the LPL gene (Gly188Glu), and this did not segregate with the FCH phenotype in the family. We conclude that in our highly selected group of FCH subjects of French Canadian descent, the XmnI and SstI RFLPs of the apoA-I gene and common functional mutations of the LPL gene resulting in complete LPL deficiency are not associated with FCH.     

Studies on the apolipoproteins and lipoproteins of cord serum

Studies on the cord serum lipid transport system were initiated to determine whether there is a correlation between decreased cord serum lipid levels and the absence or diminished level of some or all of the human serum apolipoproteins. Immunologic studies indicated the presence of all the well-characterized apolipoproteins and provided evidence that these apolipoproteins occurred primarily as distinct lipoprotein species with a paucity of association complexes or what others have termed "triglyceride-rich" lipoproteins. Quantitation of the apolipoproteins present in cord serum by electroimmunoassay yielded the following mean levels: A-I = 73O mg/liter; A-II = 410 mg/liter; apolipoprotein B = 280 mg/liter; C-I = 59 mg/liter; C-II = 32 mg/liter; C-III = 65 mg/liter; apolipoprotein D = 37 mg/liter; and apolipoprotein E = 85 mg/liter. Levels of C-I, C-II, and apolipoprotein E approached adult levels (83 to 86% of the adult levels), whereas apolipoproteins B and D were most reduced when compared to the adult concentrations, 29 and 37%, respectively. The three other apolipoproteins were present at approximately one-half the levels found in adults.     

Clinical chemistry of common apolipoprotein E isoforms

Apolipoprotein E plays a central role in clearance of lipoprotein remnants by serving as a ligand for low-density lipoprotein and apolipoprotein E receptors. Three common alleles (apolipoprotein E(2), E(3) and E(4)) give rise to six phenotypes. Apolipoprotein E(3) is the ancestral form. Common apolipoprotein E isoforms derive from nucleotide substitutions in codons 112 and 158. Resulting cysteine-arginine substitutions cause differences in: affinities for low-density lipoprotein and apolipoprotein E receptors, low-density lipoprotein receptor activities, distribution of apolipoprotein E among lipoproteins, low-density lipoprotein formation rate, and cholesterol absorption. Accompanying changes in triglycerides, cholesterol and low-density lipoprotein may promote atherosclerosis development. Over 90% of patients with familial dysbetalipoproteinaemia have apolipoprotein E(2)/E(2). Apolipoprotein E(4) may promote atherosclerosis by its low-density lipoprotein raising effect. Establishment of apolipoprotein E isoforms may be important for patients with diabetes mellitus and several non-atherosclerotic diseases. Apolipoprotein E phenotyping exploits differences in isoelectric points. Isoelectric focusing uses gels that contain pH 4-7 ampholytes and urea. Serum is directly applied, or prepurified by delipidation, lipoprotein precipitation or dialysation. Isoelectric focusing is followed by immunofixation/protein staining. Another approach is electro- or diffusion blotting, followed by protein staining or immunological detection with anti-apolipoprotein E antibodies and an enzyme-conjugated second antibody. Apolipoprotein E genotyping demonstrates underlying point mutations. Analyses of polymerase chain reaction products are done by allele-specific oligonucleotide probes, restriction fragment length polymorphism, single-stranded conformational polymorphism, the primer-guided nucleotide incorporation assay, or denaturating gradient gel electrophoresis. Detection with primers that either or not initiate amplification is performed with the amplification refractory mutation system. Disparities between phenotyping and genotyping may derive from isoelectric focusing methods that do not adequately separate apolipoprotein E posttranslational variants, storage artifacts or faint isoelectric focusing bands.     

Elevated levels of serum lipoprotein(a) and apolipoprotein(a) phenotype are not related to pre-eclampsia

BACKGROUND: Pre-eclampsia might result from a less effective invasion of trophoblast cells in the myometrium, caused by attenuated immunosuppression in the spiral arteries, resulting from inhibition of plasmin-mediated activation of transforming growth factor-beta-like substances. In vitro evidence indicates that lipoprotein(a) is capable of inhibiting plasmin-mediated activation of transforming growth factor-beta. Thus, high plasma levels of lipoprotein(a) might result in increased incidence of preeclampsia. METHODS: The patient group consisted of 39 patients with a history of pre-eclampsia in a previous pregnancy: Forty-seven women without pre-eclampsia in their history and matched for age were the control group. All participants gave their informed consent. In both the patient and control group blood pressure, CRP, urinalysis, cholesterol, HDL-cholesterol, triglycerides, lipoprotein(a) level and apolipoprotein(a) phenotype were determined. RESULTS: None of the participants had elevated CRP levels, excluding acute phase related elevations of lipoprotein(a). Proteinuria was present in 33% of patients and in 11% of controls (p=0.01). However, no relation was observed between proteinuria and Lp(a) level. Median Lipoprotein(a) levels in both groups were equal (300 mg/l vs. 275 mg/l; p=0.48), as well as the apo(a) phenotype distribution in both groups. CONCLUSIONS: Lipoprotein(a) and apolipoprotein(a) phenotype do not contribute significantly to the pathogenesis of pre-eclampsia.     

Hypocalcemia and hyperphosphatemia due to primary hypoparathyroidism in a six-month-old kitten

The question why low-density lipoprotein (LDL) stranded in the subendothelium of arteries should acquire the proinflammatory properties that initiate and sustain atherogenesis has puzzled researchers for decades. The most popular concept contends that oxidative processes are crucial because oxidized LDL (ox-LDL) produced in vitro has atherogenic properties and small amounts of it are found in atherosclerotic lesions. Recently, a possible role for vascular infections has also been considered because infectious agents, in particular Chlamydia pneumoniae, are sometimes present in the lesions. Here, evidence is summarized for a different concept of atherogenesis, which evolves from the fact that nonoxidative, enzymatic degradation of LDL transforms the lipoprotein to an atherogenic moiety. Our group proposes that enzymatically degraded LDL (E-LDL) initiates and sustains atherosclerosis through its capacity to activate complement and macrophages. These processes are initially meaningful because they enable the stranded lipoprotein to be removed from the vessel wall, but they become harmful when the cholesterol removal system is overloaded. A novel type of chronic inflammation then ensues producing the characteristic pathology of the atherosclerotic lesion.     

Is the development of adenoma and carcinoma in proximal colon related to apolipoprotein E phenotype?

BACKGROUND & AIMS: Alterations in plasma lipoprotein levels and bile acid metabolism observed in patients with colorectal adenoma and carcinoma may reflect a genetic background predisposing to altered lipid metabolism and tumors. This study was designed to determine whether the polymorphism of apolipoprotein E, one of the key regulatory proteins in cholesterol metabolism, is associated with proximal or distal colonic neoplasia. METHODS: Apolipoprotein E phenotype was determined in 135 patients with colorectal adenoma, 122 patients with colorectal carcinoma, and 199 randomly selected control subjects. RESULTS: The frequency of the epsilon 4 allele of apolipoprotein E was low (0.075 and 0.073) in patients with proximal adenoma and those with carcinoma, respectively, compared with the control subjects (0.181) (P < 0.05). In patients with distal tumors, there was no alteration in epsilon 4 frequency. In all subjects with the epsilon 4 allele compared with subjects without epsilon 4, the odds ratio for proximal adenoma was 0.36 (95% confidence interval, 0.14-0.89), and the odds ratio for proximal carcinoma was 0.35 (95% confidence interval, 0.14-0.86). CONCLUSIONS: The data suggest that the epsilon 4 allele of apolipoprotein E provides protection from the development of adenoma and carcinoma of the proximal colon. These results support the theory that there are common susceptibility genes modulating the susceptibility to external carcinogenic factors.     

Apolipoprotein A-I(Zavalla) (Leu159-->Pro): HDL cholesterol deficiency in a kindred associated with premature coronary artery disease

We investigated the molecular defect causing high density lipoprotein cholesterol (HDL-C) deficiency in a male proband and his family members. Amplification and sequencing of genomic DNA disclosed a novel base-pair substitution at residue 159 in the apolipoprotein (apo) A-I gene. This substitution resulted in the loss of an AviII restriction site and a predicted substitution of leucine with proline at residue 159. Restriction enzyme analysis demonstrated absence of the AviII site in 19 of 40 biological family members. Compared with familial controls, subjects with the apoA-I(Zavalla) variant had reduced HDL-C (1.16 versus 0.27 mmol/L, P<0.0001), apoA-I (38.7 versus 124.4 mg/dL, P<0.0001), and apoA-II (14.3 versus 19.0 mg/dL, P<0.0001) levels. Two subjects who have developed coronary artery disease to date possess additional cardiovascular risk factors. Other heterozygotes for apoA-I(Zavalla) are presently without symptomatic coronary artery disease. This study identifies a monogenic cause of hypoalphalipoproteinemia, with the single base-pair substitution having a dominant effect on the low HDL-C phenotype. In addition, it extends recent observations that HDL-C deficiency states may be more prone to the development of premature coronary artery disease when accompanied by additional cardiovascular risk factors.     

Psychological and socio-medical aspects of HIV/AIDS: a reflection on publications in AIDS care (1989-1995)

Cardiovascular disease is the leading cause of death in non-insulin dependent diabetes mellitus and first degree relatives of such patients are at increased risk of developing diabetes and cardiovascular disease. The aim of the present study was to determine whether lipid abnormalities occur in normoglycaemic relatives of non-insulin dependent diabetic patients. Cholesterol, triglycerides, apolipoprotein A-I and apolipoprotein B concentrations were measured in serum; the lipoprotein fractions very low density, intermediate density, low density and high density lipoprotein were prepared by sequential flotation ultracentrifugation and their composition investigated. The groups were matched for age, sex and blood glucose concentrations although the relatives (n = 126) were more insulin resistant as determined using the homeostasis model assessment method [1.9 (0.8-9.0) vs 1.6 (0.4-4.9) mmol/mU per l (mean [95% confidence intervals]); p < 0.001] and had greater body mass indices [26.6 (4.1) vs 24.8 (3.9) (mean [S.D.]); p = 0.001] than control subjects (n = 126). Relatives had higher serum apolipoprotein B concentrations than control subjects [0.9 (0.3) vs 0.8 (0.3) g/l, p = 0.02) and lower serum apolipoprotein A-I concentrations (1.4 (0.3) vs 1.5 (0.3), p = 0.02). In multivariate linear regression analysis of all subjects log insulin resistance (p = 0.0001), age (p = 0.002) and waist:hip ratio (p = 0.01) were independent predicators of apolipoprotein B concentrations while waist:hip ratio (p < 0.001) and smoking status (p = 0.002) were independent predictors of apolipoprotein A-I concentrations. Lipoprotein composition (measured in a subgroup of 76 control subjects and 88 relatives), serum cholesterol and serum triglyceride concentrations did not differ between the groups. We conclude that atherogenic apolipoprotein abnormalities occur in normoglycaemic relatives of non-insulin dependent diabetic patients.     

Evidence for multiple open and inactivated states of the hKv1.5 delayed rectifier

We previously found in human blood a fraction of low-density lipoprotein (LDL) that is characterized by a reduced content of sialic acid. Desialylated LDL also has a low neutral carbohydrate level, decreased content of major lipids, small size, high density, increased electronegative charge and altered tertiary apolipoprotein B structure. Unlike native LDL, this fraction of desialylated (multiple-modified) LDL induces the accumulation of lipids in smooth muscle cells cultured from unaffected human aortic intima, i.e. it exhibits atherogenic properties. In this study, we attempted to elucidate the mechanism of desialylation and other changes in the multiple-modified LDL by investigating the possibility of LDL modification by different cells and the blood plasma. A 24-h incubation at 37 degrees C of lipoprotein with intact endotheliocytes, hepatocytes, macrophages and smooth muscle cells or cell homogenates did not cause alterations either in the physical properties or in the chemical composition of native LDL. On the other hand, a significant fall in the lipoprotein sialic acid level was observed already after a 1-h incubation of native LDL with an autologous plasma-derived serum. While LDL sialic acid level continuously decreased, LDL became capable of inducing the accumulation of total cholesterol in the smooth muscle cells cultured from unaffected human aortic intima after 3 h of incubation. Starting from the sixth hour of LDL incubation with serum, a steady decrease in the lipoprotein lipid content was observed as well as the related reduction of LDL size. Following 36 h of incubation, an increase in the negative charge of lipoprotein particles was also seen. Prolonged incubation of LDL with plasma-derived serum (48 and 72 h) leads to the loss of alpha-tocopherol by the LDL as well as to an increase in LDL susceptibility to copper oxidation and to accumulation of cholesterol covalently bound to apolipoprotein B, a marker of lipoperoxidation. Degradation of apolipoprotein B starts within the same period of time. Hence, desialylation of LDL particles represents one of the first or the primary act of modification which is, apparently, a sufficient prerequisite for the development of atherogenic properties. Subsequent modifications just enhance the atherogenic potential of LDL. The loss of sialic acid by LDL occurred at neutral pH and was not inhibited by the sialidase inhibitor 2,3-dehydro-2-deoxy-N-acetylneuraminic acid. The [3H]sialic acid removed from LDL was not found in free form, but in the plasma fraction precipitated by trichloroacetic acid. These data along with the fact that cytidine-5'-triphosphate inhibited LDL desialylation suggest that enzymes close to sialyltransferases play a role in this process. Thus, this study demonstrated that the LDL modification processes imparting atherogenic properties to this lipoprotein can take place in human blood plasma. Multiple modification of LDL is a cascade of successive changes in the lipoprotein particle: desialylation, loss of lipids, reduction in particle size, increase of its electronegative charge and peroxidation of lipids.     

Lipid and lipoprotein profiles in children with familial hypercholesterolaemia: effects of therapy

In 71 children with familial hypercholesterolaemia the effect of dietary and/or medical treatment was evaluated. Initial total cholesterol and low density lipoprotein (LDL)-cholesterol levels were significantly lower in children who were consecutively treated by diet (Step-One-Diet) than in those who received additional medication. By dietary treatment, the median total cholesterol level (236.5 mg/dl; range 210-510 mg/dl) was reduced by 7.4% and the median LDL-cholesterol level (162 mg/dl; range 126-423 mg/dl) by 9.9%. By dietary and medical therapy, the median total cholesterol level (330 mg/dl; range 270-424 mg/dl) was reduced by 29.7% and the median LDL-cholesterol level (263 mg/dl; 192-333 mg/dl) by 25.9%. High density lipoprotein (HDL)-cholesterol and HDL 3 remained unchanged. HDL 2 showed a significant decrease of 15.6% up to 27 mg/dl (13-42 mg/dl) on medical treatment. Apolipoprotein A I levels did not change during therapy. Initial apolipoprotein B levels were significantly higher in children who were treated by diet and medication and were reduced by 28.9% by combined therapy. In 28 patients (39.4%) an excess of lipoprotein (a) was detected. Regarding the apolipoprotein E phenotype, 32.2% of the patients carried the risk gene epsilon4 in a hetero- or homozygous form. CONCLUSION: Early dietary and/or medical treatment in hypercholesterolaemic children significantly ameliorates the lipoprotein status. The pretherapy lipoprotein status seems to prognosticate the effectiveness of therapy.     

Lipoprotein Lp(a): effects of allelic variation at the LPA locus

Concentrations of the lipoprotein Lp(a) vary widely in healthy individuals, but in many studies, high concentrations are strongly associated with cardiovascular disease. On the basis of lipid and protein composition, Lp(a) is a variant of the atherogenic low-density lipoprotein but differs in possessing the unique apolipoprotein(a) [apo(a)]. Lp(a) concentrations are controlled at the level of biosynthesis of the apo(a) protein, which is encoded by the LPA locus, and allelic differences at LPA are responsible for the bulk of variation in Lp(a) phenotype. In this article we describe several aspects of allelic variation at LPA reported in studies of human and baboons, including (1) polymorphisms for protein size, (2) families of alleles having distinct relationships between apo(a) size and Lp(a) concentration, (3) sequence polymorphisms, (4) a group of alleles whose protein products have a multibanded phenotype, and (5) allelic diversity of null phenotype alleles (whose protein products are not detected in the plasma). The data make clear that no single aspect of allelic variation at LPA is sufficient to fully explain the genetic control of Lp(a).