Accumulation of lipoprotein fractions and subfractions in the arterial wall, determined in an in vitro perfusion system

Low-Density Lipoprotein Subfractions and the Prevalence of Silent Lacunar Infarction in Subjects with Essential Hypertension

Recent lipid research has focused on low-density lipoprotein (LDL) subfractions as new markers for cardiovascular risk. However, the clinical significance of measurement of LDL subfractions in subjects with essential hypertension is yet to be established. We studied the association between the prevalence of silent lacunar infarction (SLI) and LDL subfractions in patients with essential hypertension. We performed brain MRI to detect SLI and measured LDL subfractions in 100 asymptomatic non-diabetic middle-aged subjects with essential hypertension (mean age, 62 years). We fractionated LDL into three parts, LDL-1, LDL-2, and LDL-3, with LDL-3 being the oxidized subfraction. Of the 100 study subjects, 24 (24%) had one or more SLIs, while the remaining 76 (76%) were considered as a non-SLI group. The LDL-3 levels were significantly higher in the SLI group than in the non-SLI group (8.3 +/- 4.4 mg/dl vs. 6.3 +/- 2.0 mg/dl, p = 0.006). Multiple logistic regression analysis showed that LDL-3 levels alone were an independent predictor of SLI (odds ratio [OR]: 1.380; 95% confidence interval [CI]: 1.113-1.663; p = 0.003). When subjects were divided into quartiles based on LDL-3 levels, the prevalence of SLI was significantly higher in the highest LDL-3 level group than in the lowest LDL-3 level group (p = 0.0036). The present study suggests that LDL-3 levels are associated with the prevalence of SLI in subjects with essential hypertension.

Metabolic syndrome: the danger signal in atherosclerosis

Atherosclerosis is a chronic inflammatory disease characterized by infiltration of blood vessels by lipids and leukocytes. There is a growing body of evidence that among risk factors that promote atherosclerosis, the metabolic syndrome is a powerful and prevalent predictor of cardiovascular events. The systemic inflammatory process associated with the metabolic syndrome has numerous deleterious effects that promote plaque activation, which is responsible for clinical events. Interactions between the innate immune system with lipid-derived products seem to play a major role in the pathophysiology of atherosclerosis in relation with the metabolic syndrome. The multiple links among adipose tissue, the vascular wall, and the immune system are the topics of this review, which examines the roles of oxidized low-density lipoprotein, inflammatory cytokines, and adipokines in triggering and perpetuating a danger signal response that promotes the development of atherosclerosis. Furthermore, therapeutic options that specifically target the metabolic syndrome components are reviewed in light of recent developments.

Increased small low-density lipoprotein particle number: a prominent feature of the metabolic syndrome in the Framingham Heart Study

BACKGROUND

Levels of LDL cholesterol (LDL-C) are frequently not elevated in individuals with the metabolic syndrome (MetSyn). However, the atherogenic potential of LDL may depend on the number and size of LDL particles in addition to the cholesterol content of LDL.

METHODS AND RESULTS

We examined the sex-specific cross-sectional relations of small LDL particle number (determined by nuclear magnetic resonance spectroscopy) to the presence of MetSyn and its components in 2993 Framingham Heart Study participants (mean age, 51 years; 53% women) without cardiovascular disease (CVD) and the relations of small LDL particle number to CVD incidence in people with MetSyn. The MetSyn (> or =3 of 5 traits as defined by the National Cholesterol Education Adult Treatment Panel III) was present in 27% of men and 17% of women. In both sexes, small LDL particle number increased from 0 to 5 MetSyn traits, a pattern partly accounted for by strong correlations between small LDL particle number and serum triglycerides (r=0.61, P<0.0001) and HDL-C (r=-0.55, P<0.0001). Compared with participants without the MetSyn, those with the MetSyn had a higher CVD event rate. However, among participants with the MetSyn, CVD rates were similar for groups with an elevated versus a lower number of small LDL particles (defined by the sex-specific median).

CONCLUSIONS

Small LDL particle number is elevated in the MetSyn, increases with the number of MetSyn components, and most prominently is correlated with triglycerides and HDL-C. Whereas increased small LDL particle number identified the MetSyn with high sensitivity, a higher small LDL particle number was not associated with greater CVD event rates in people with the MetSyn.

Influence of dietary carbohydrate and fat on LDL and HDL particle distributions

Variations in the size and density distributions of low-density lipoprotein (LDL) and high-density lipoprotein (HDL) particles have been related to risk for cardiovascular disease. In particular, increased levels of small, dense LDL particles, together with reduced levels of large HDL and increases in small HDL, are integral features of the atherogenic dyslipidemia found in patients with insulin resistance, obesity, and metabolic syndrome. Increased dietary carbohydrates, particularly simple sugars and starches with high glycemic index, can increase levels of small, dense LDL and HDL, primarily by mechanisms that involve increasing plasma triglyceride concentrations. Low-carbohydrate diets may have the opposite effects. Diets with differing fatty acid composition can also influence LDL and HDL particle distributions.

Distribution of LDL particle size in a population-based sample of children and adolescents and relationship with other cardiovascular risk factors

BACKGROUND

Smaller, denser LDL particles are associated with an increased risk for cardiovascular diseases (CVD). In youths, data on the distribution of LDL particle size and on its association with other CVD risk factors are limited.

METHODS

We determined LDL peak particle size by nondenaturing 2%-16% gradient gel electrophoresis in a representative sample of 2249 youths 9, 13, and 16 years of age who participated in a school-based survey conducted in 1999 in the province of Quebec, Canada. Standardized clinical measurements and fasting plasma lipid, glucose, and insulin concentrations were available.

RESULTS

The LDL peak particle size distribution was gaussian. The 5th, 50th (median), and 95th percentiles by age and sex were 255.5-258.6, 262.1-263.2, and 268.1-269.5 A, respectively. The prevalence of the small, dense LDL phenotype (LDL peak particle size <or=255 A) was 10% in participants with insulin resistance syndrome (IRS), in contrast to 1% in those without IRS. In a multiple regression analysis, the association of LDL size with other CVD risk factors [apolipoprotein B, HDL-cholesterol (HDL-C), triglyceride (TG), and insulin concentrations, and body mass index] was strongest with TG and HDL-C concentrations: a 1 SD increase in log(e)-transformed TG concentration was associated with a 1.2 A reduction in LDL size, and a 1 SD increase in HDL-C was associated with a 1.1 A increase in LDL size.

CONCLUSIONS

Although the small, dense LDL phenotype is less prevalent in youths than adults, its prevalence is clearly increased in childhood IRS. Metabolic correlates of LDL size are similar in youths and adults.

Clinical significance of the physicochemical properties of LDL in type 2 diabetes

Atherosclerosis is the leading cause of death in type 2 diabetes. LDL cholesterol and atherosclerosis are related, both in healthy people and those with diabetes; however, people with diabetes are more prone to atheroma, even though their LDL cholesterol levels are similar to those in their non-diabetic peers. This is because LDL particles are modified in the presence of diabetes to become more atherogenic. These modifications include glycation in response to high plasma glucose levels; oxidative reactions mediated by increased oxidative stress; and transfer of cholesterol ester, which makes the particles smaller and denser. The latter modification is strongly associated with hypertriglyceridaemia. Oxidatively and non-oxidatively modified LDL is involved in plaque formation, and may thus contribute to the accelerated atherosclerosis. This review discusses the techniques currently used to determine the physicochemical properties of LDL, and examines the evidence that modification of these properties plays a role in the accelerated atherosclerosis associated with type 2 diabetes.

Low-density lipoprotein size and subclasses are markers of clinically apparent and non-apparent atherosclerosis in type 2 diabetes

The atherogenic lipoprotein phenotype is characterized by an increase in plasma triglycerides, a decrease in high-density lipoprotein (HDL), and the prevalence of small, dense low-density lipoprotein (LDL) particles. The present study investigated the clinical significance of LDL size and subclasses as markers of atherosclerosis in diabetes type 2. Thirty-eight patients with type 2 diabetes, total cholesterol of less than 6.5 mmol/L, and hemoglobin A1c (HbA1c) of less than 9% were studied. Median age was 61 years, mean (+/-SD) body mass index 29 +/- 4.3 kg/m2 , and mean HbA1c 7.1 +/- 0.9 %. Laboratory parameters included plasma lipids and lipoproteins, lipoprotein (a), apolipoprotein (apo) A-I, apo B-100, apo C-III, and high-sensitivity C-reactive protein. Low-density lipoprotein size and subclasses were measured by gradient gel electrophoresis and carotideal intima media thickness (IMT) by duplex ultrasound. By factor analysis, 10 out of 21 risk parameters were selected: age, body mass index, systolic blood pressure, smoking (in pack-years), HbA1c, high-sensitivity C-reactive protein, lipoprotein (a), LDL cholesterol, HDL cholesterol, and LDL particle size. Multivariate analysis of variance of these 10 risk parameters identified LDL particle size as the best risk predictor for the presence of coronary heart disease (P = .002). Smaller LDL particle size was associated with an increase in IMT (P = .03; cut-off >1 mm). Within the different lipid parameters (total cholesterol, LDL cholesterol, HDL cholesterol, triglycerides, apo B, apo A-I, apo C-III, LDL particle size), LDL particle size was most strongly associated with the presence of coronary heart disease (P = .002) and IMT (P = .03). It is concluded that LDL size is the strongest marker for clinically apparent as well as non-apparent atherosclerosis in diabetes type 2.

Measurement of Small Dense Low-density Lipoprotein Particles

Low density lipoprotein (LDL) particles are heterogeneous with respect to their size, density and lipid composition. Among LDL particles, the smaller and denser LDL particles [small dense (sd) LDL] are more atherogenic and the sd LDL phenotype is strongly associated with development of coronary heart disease. Here we will review various methods for measurement of sd LDL. Although ultracentrifugation, nuclear magnetic resonance (NMR) spectroscopy and gradient-gel electrophoresis (GGE) are usually employed for the measurement of sd LDL, such methods are either too laborious or expensive for general clinical use. We recently established a simple precipitation method for the quantification of sd LDL. This method is applicable to routine clinical use and allows the rapid measurement of a large number of samples.

Management of diabetic dyslipidemia: need for reappraisal of the goals

Cardiovascular disease is the leading cause of mortality among people with diabetes mellitus, accounting for 70% of all deaths. As the prevalence of diabetes increases significantly worldwide, greater attention must be focused on preventing cardiovascular events in this group. One contributor to this increased event rate is the characteristic pattern of dyslipidemia in diabetic patients, consisting of elevated serum triglyceride levels, decreased high-density lipoprotein levels, and an increased proportion of small, dense, low-density lipoproteins. Several pharmacologic agents have been used to treat this dyslipidemia including HMG-CoA reductase inhibitors, fibric acid derivatives, niacin (nicotinic acid), thiazolidinediones, and fish oils, as well as other non-pharmacologic measures. Currently, the most extensive data for a reduction in cardiovascular events in patients with diabetes exist for HMG-CoA reductase inhibitors. The results of these trials indicate that HMG-CoA reductase inhibitor therapy should be considered for all patients with diabetes at sufficient risk for cardiovascular events, regardless of serum low-density lipoprotein-cholesterol level. Several ongoing trials of various pharmacologic agents should help clarify the role of these agents alone and in combination with HMG-CoA reductase inhibitors in the management of diabetic dyslipidemia.

Mechanism of lipoprotein retention by the extracellular matrix

PURPOSE OF REVIEW

Considerable evidence suggests that the subendothelial retention of atherogenic lipoproteins is a key early step in atherogenesis. In humans and experimental animals, elevated levels of plasma lipoproteins are associated with increased atherosclerosis, and lipoproteins with higher affinity for arterial proteoglycans are more atherogenic. Here we discuss the molecular mechanisms underlying lipoprotein retention in the arterial wall and how this interaction can be modulated.

RECENT FINDINGS

Functional proteoglycan binding sites in lipoproteins containing apolipoprotein B have been identified and shown to have atherogenic potential in vivo. In addition to apolipoprotein B, novel bridging molecules, those that can interact with both proteoglycans and lipoproteins, have been identified that mediate the retention of atherogenic particles in the vessel wall. The interaction between lipoproteins and proteoglycans can be enhanced by the modification of lipoproteins in the circulation and in the arterial wall, by alterations in the subendothelium, and by changes in proteoglycan synthesis that result in a more atherogenic profile. The retention of atherogenic lipoproteins is a potential target for therapies to reverse atherosclerosis, and in-vitro studies have identified compounds that decrease the affinity of proteoglycans for lipoproteins.

SUMMARY

Considerable progress has been made in understanding the association between lipoproteins and cardiovascular disease. This review highlights the importance of the interaction between lipoproteins and the arterial matrix.

Dyslipidemia in type 2 diabetes

Type 2 diabetes mellitus is associated with a cluster of lipid abnormalities:elevated plasma triglycerides, reduced high-density lipoprotein cholesterol, and smaller and denser low-density lipoproteins,which have been associated with an increased risk of cardiovascular disease. Insulin resistance may contribute to dyslipidemia associated with type 2 diabetes by increasing hepatic secretion of large,triglyceride-rich very low-density lipoprotein particles and by impairing the clearance of lipoprotein particles from plasma. Lifestyle interventions may be effective in improving the diabetic dyslipidemia syndrome. For patients who do not respond to lifestyle changes, pharmacologic therapies (lipid-lowering medications and anti-diabetic agents) are available. Clinical trials demonstrate that the use of such pharmaceutics to treat diabetic dyslipidemia concomitantly reduces the risk of coronary artery disease.

Lipids and lipoproteins in patients with type 2 diabetes

Insulin resistance and type 2 diabetes are associated with a clustering of interrelated plasma lipid and lipoprotein abnormalities, which include reduced HDL cholesterol, a predominance of small dense LDL particles, and elevated triglyceride levels. Each of these dyslipidemic features is associated with an increased risk of cardiovascular disease. Increased hepatic secretion of large triglyceride-rich VLDL and impaired clearance of VLDL appears to be of central importance in the pathophysiology of this dyslipidemia. Small dense LDL particles arise from the intravascular processing of specific larger VLDL precursors. Typically, reduced plasma HDL levels in type 2 diabetes are manifest as reductions in the HDL(2b) subspecies and relative or absolute increases in smaller denser HDL(3b) and HDL(3c). Although behavioral interventions such as diet and exercise can improve diabetic dyslipidemia, for most patients, pharmacological therapy is needed to reach treatment goals. There are several classes of medications that can be used to treat lipid and lipoprotein abnormalities associated with insulin resistance and type 2 diabetes, including statins, fibrates, niacin, and thiazolidinediones. Clinical trials have shown significant improvement in coronary artery disease after diabetic dyslipidemia treatment.

Genetics of LDL particle heterogeneity

Substantial evidence exists suggesting that small, dense LDL particles are associated with an increased risk of coronary heart disease. This disease-related risk factor is recognized to be under both genetic and environmental influences. Several studies have been conducted to elucidate the genetic architecture underlying this trait, and a review of this literature seems timely. The methods and strategies used to determine its genetic component and to identify the genes have greatly changed throughout the years owing to the progress made in genetic epidemiology and the influence of the Human Genome Project. Heritability studies, complex segregation analyses, candidate gene linkage and association studies, genome-wide linkage scans, and animal models are all part of the arsenal to determine the susceptibility genes. The compilation of these studies clearly revealed the complex genetic nature of LDL particles. This work is an attempt to summarize the growing evidence of genetic control on LDL particle heterogeneity with the aim of providing a concise overview in one read.

Metabolic abnormalities: triglyceride and low-density lipoprotein

Increased plasma triglyceride and reduced high-density lipoprotein cholesterol are key features of the metabolic syndrome. Although elevated low-density lipoprotein cholesterol is not an integral characteristic of this syndrome, there is commonly an increase in the proportion of small, dense low-density lipoprotein particles. Together, these abnormalities constitute the atherogenic dyslipidemia of the metabolic syndrome. This article reviews the pathophysiology of altered triglyceride and low-density lipoprotein metabolism in the metabolic syndrome, outlines the relationship of these lipoprotein abnormalities to increased risk of coronary heart disease,and highlights the application of this information to clinical practice. The role of reduced high-density lipoprotein in the metabolic syndrome is discussed elsewhere in this issue.

Abdominal obesity and dyslipidemia in the metabolic syndrome: importance of type 2 diabetes and familial combined hyperlipidemia in coronary artery disease risk

Regional body fat distribution has an important influence on metabolic and cardiovascular risk factors. Increased abdominal (visceral) fat accumulation is a risk factor for coronary artery disease (CAD), dyslipidemia, hypertension, stroke, and type 2 diabetes. The recent emphasis on treatment of the dyslipidemia of the metabolic syndrome (hypertriglyceridemia, reduced high-density lipoprotein, and increased small, dense low-density lipoprotein particle number) has compelled practitioners to consider lipid-lowering therapy in a greater number of their patients, as one in two individuals over age 50 has the metabolic syndrome. Individuals with the metabolic syndrome typically have normal low-density lipoprotein cholesterol levels, and current lipid-lowering guidelines may underestimate their cardiovascular risk. Two subgroups of patients with the metabolic syndrome are at particularly high risk for premature CAD. One, individuals with type 2 diabetes, accounts for 20-30% of early cardiovascular disease. The second, familial combined hyperlipidemia, accounts for an additional 10-20% of premature CAD. Familial combined hyperlipidemia is characterized by the metabolic syndrome in addition to a disproportionate elevation of apolipoprotein B levels. The measurement of fasting glucose and apolipoprotein B, in addition to the fasting lipid profile, can help to estimate CAD risk in patients with the metabolic syndrome.

Hepatic ABCG5 and ABCG8 Overexpression Increases Hepatobiliary Sterol Transport but Does Not Alter Aortic Atherosclerosis in Transgenic Mice

The individual roles of hepatic versus intestinal ABCG5 and ABCG8 in sterol transport have not yet been investigated. To determine the specific contribution of liver ABCG5/G8 to sterol transport and atherosclerosis, we generated transgenic mice that overexpress human ABCG5 and ABCG8 in the liver but not intestine (liver G5/G8-Tg) in three different genetic backgrounds: C57Bl/6, apoE-KO, and low density lipoprotein receptor (LDLr)-KO. Hepatic overexpression of ABCG5/G8 enhanced hepatobiliary secretion of cholesterol and plant sterols by 1.5-2-fold, increased the amount of intestinal cholesterol available for absorption and fecal excretion by up to 27%, and decreased the accumulation of plant sterols in plasma by approximately 25%. However, it did not alter fractional intestinal cholesterol absorption, fecal neutral sterol excretion, hepatic cholesterol concentrations, or hepatic cholesterol synthesis. Consequently, overexpression of ABCG5/G8 in only the liver had no effect on the plasma lipid profile, including cholesterol, HDL-C, and non-HDL-C, or on the development of proximal aortic atherosclerosis in C57Bl/6, apoE-KO, or LDLr-KO mice. Thus, liver ABCG5/G8 facilitate the secretion of liver sterols into bile and serve as an alternative mechanism, independent of intestinal ABCG5/G8, to protect against the accumulation of dietary plant sterols in plasma. However, in the absence of changes in fractional intestinal cholesterol absorption, increased secretion of sterols into bile induced by hepatic overexpression of ABCG5/G8 was not sufficient to alter hepatic cholesterol balance, enhance cholesterol removal from the body or to alter atherogenic risk in liver G5/G8-Tg mice. These findings demonstrate that overexpression of ABCG5/G8 in the liver profoundly alters hepatic but not intestinal sterol transport, identifying distinct roles for liver and intestinal ABCG5/G8 in modulating sterol metabolism.

Plasma clearance of human low-density lipoprotein in human apolipoprotein B transgenic mice is related to particle diameter

To test for intrinsic differences in metabolic properties of low-density lipoprotein (LDL) as a function of particle size, we examined the kinetic behavior of 6 human LDL fractions ranging in size from 251 to 265 A injected intravenously into human apolipoprotein (apo) B transgenic mice. A multicompartmental model was formulated and fitted to the data by standard nonlinear regression using the Simulation, Analysis and Modeling (SAAM II) program. Smaller sized LDL particles (251 to 257 A) demonstrated a significantly slower fractional catabolic rate (FCR) (0.050 +/- 0.045 h(-1)) compared with particles of larger size (262 to 265 A) (0.134 +/- -0.015 h(-1), P <.03), and there was a significant correlation between FCR and the peak LDL diameter of the injected fractions (R(2) =.71, P <.034). The sum of the equilibration parameters, k(2,1) and k(1,2), for smaller LDL (0.255 h(-1) and 0.105 h(-1), respectively) was significantly smaller than that for larger LDL (0.277 h(-1) and 0.248 h(-1), respectively; P <.01), indicative of slower intravascular-extravascular exchange for smaller LDL. Therefore in this mouse model, smaller LDL particles are cleared more slowly from plasma than larger LDL and are exchanged more slowly with the extravascular space. This might be due to compositional or structural features of smaller LDL that lead to retarded clearance.

Unesterified plant sterols and stanols do not affect LDL electrophoretic characteristics in hypercholesterolemic subjects

The extent to which sterols and stanols modulate LDL particle size is unknown. We examined the effects of supplementation with unesterified plant sterols and stanols on several LDL electrophoretic characteristics. Healthy hypercholesterolemic subjects (n = 14) consumed each of four experimental diets contained plant sterols (S), plant stanols (SN), a 50:50 mixture of sterols and stanols (SSN), or cornstarch (control) in a randomized crossover design. The butter component of the diet was blended with unesterified sterols and stanols at a dose of 1.8 g/d. The LDL particles were characterized by polyacrylamide gradient gel electrophoresis of whole plasma. LDL cholesterol (LDL-C) concentrations decreased by 8.8, 13.6, and 13.1% in the S, SN, and SSN groups, respectively (P < 0.01) with a significant increase of 4.3% in the control group. None of the treatments with sterols and stanols induced significant changes in LDL peak particle diameter or in the cholesterol levels of the small LDL subfraction (<25.5 nm). The reduction in plasma LDL-C levels with SN consumption was due mainly to a decrease (P < 0.05) in the concentration of cholesterol in the large subfraction (>26.0 nm). The significant reduction in plasma LDL-C concentrations by sterol and stanol consumption in subjects was not paralleled by any beneficial changes in LDL electrophoretic characteristics.

Group V sPLA2 hydrolysis of low-density lipoprotein results in spontaneous particle aggregation and promotes macrophage foam cell formation

OBJECTIVE

Secretory phospholipase A2 (sPLA2) enzymes hydrolyze the sn-2 fatty acyl ester bond of phospholipids to produce a free fatty acid and a lysophospholid. Group V sPLA2 is expressed in cultured macrophage cells and has high affinity for phosphatidyl choline-containing substrates. The present study assesses the presence of group V sPLA2 in human and mouse atherosclerotic lesions and its activity toward low-density lipoprotein (LDL) particles.

METHODS AND RESULTS

Group V sPLA2 was detected in human and mouse atherosclerotic lesions by immunohistochemical staining. Electron microscopic analysis showed that mouse group V sPLA2-modified LDL is significantly smaller (mean diameter+/-SEM=25.3+/-0.25 nm) than native LDL (mean diameter+/-SEM=27.7+/-0.29 nm). Hydrolysis by group V sPLA2 induced spontaneous particle aggregation; the extent of aggregation was directly proportional to the degree of LDL hydrolysis. Group V sPLA2 modification of LDL led to enhanced lipid accumulation in cultured mouse peritoneal macrophage cells.

CONCLUSIONS

Group V sPLA2 may play an important role in promoting atherosclerotic lesion development by modifying LDL particles in the arterial wall, thereby enhancing particle aggregation, retention, and macrophage uptake.

Associations of low density lipoprotein particle composition with atherogenicity

PURPOSE OF REVIEW

A growing body of data suggests that in addition to LDL-cholesterol concentrations, compositional properties of LDL, including size and fatty acid composition, are important in determining the relative degree of atherogenicity. This review examines current research in this field to evaluate which properties of LDL may most directly influence the risk of coronary heart disease.

RECENT FINDINGS

The presence of small dense LDL has been correlated with an increased risk of coronary heart disease, but this has not been shown to be fully independent of related factors such as elevated plasma triacylglycerol concentrations. An increased susceptibility of small dense LDL to in-vitro oxidation has also been demonstrated, but its importance to coronary heart disease risk has not been established. Other studies have found that the presence of enlarged LDL, modified (oleate enriched) fatty acyl composition of LDL, and higher numbers of LDL particles in plasma also are endpoints associated with an increased risk of coronary heart disease.

SUMMARY

LDL size may indicate a metabolic condition associated with increased CHD risk as opposed to the direct promotion of atherosclerosis by specific particle types of LDL. In most claims of detrimental effects of small dense LDL, neither LDL particle concentrations nor the fatty acid composition of the particles were established, both factors being important in contributing to the atherogenic potential of LDL. The predisposition to premature coronary heart disease cannot currently be objectively assigned to any one type of LDL particle.

A novel and simple method for quantification of small, dense LDL

A preponderance of small, dense (sd) LDL is strongly associated with the development of coronary heart disease, but the method for the measurement of sd LDL is too laborious for clinical use. We report a simple method for the quantification of sd LDL that is applicable to an autoanalyzer. This method consists of two steps: first, to precipitate the lipoprotein of density (d) <1.044 g/ml using heparin-magnesium; and second, to measure LDL-cholesterol in the supernatant by the homogeneous method or apolipoprotein B (apoB) by an immunoturbidometric assay. The cholesterol and apoB values obtained by the precipitation method (45 +/- 26 and 33 +/- 20 mg/dl, respectively) were similar to those obtained in the lipoprotein (d = 1.044-1.063) separated by ultracentrifugation (42 +/- 22 and 31 +/- 17 mg/dl, respectively), and there was an excellent correlation between the two methods for sd LDL-cholesterol (y = 1.05X + 1, r = 0.88, n = 69) and apoB (y = 1.07X, r = 0.90). Sd LDL values had a significant inverse correlation with LDL size. A high correlation was found between sd LDL-cholesterol and apoB values (r = 0.94). Sd LDL value was related to triglyceride, apoB, and LDL-cholesterol, but not to the buoyant LDL level. These results suggest that this precipitation method is a simple and rapid method for the measurement of sd LDL concentration.

Associations of LDL size with in vitro oxidizability and plasma levels of in vivo oxidized LDL in Type 2 diabetic patients

AIMS

Oxidative modification of low-density lipoprotein (LDL) is believed to be a key step in the genesis of atherosclerotic lesions. The presence of small, dense LDL is associated with accelerated atherosclerosis and is common in diabetic patients. The aim of this study was to investigate the relationship of in vitro LDL oxidizability and circulating in vivo oxidized LDL with LDL particle size in Type 2 diabetic patients and healthy control subjects.

SUBJECTS AND METHODS

The study group consisted of 58 elderly well controlled Type 2 diabetic patients and 58 control subjects with normal glucose metabolism. LDL particle size was measured by high-performance gel-filtration chromatography. In vitro oxidizability of LDL was measured by monitoring conjugated diene formation and plasma levels of circulating oxidized LDL were determined by ELISA.

RESULTS

In vitro susceptibility of LDL to oxidation was not related to plasma levels of in vivo oxidized LDL, nor to LDL particle size. In the diabetic patients, but not in the control group, an inverse relation between LDL size and in vivo oxidized LDL was observed (r=-0.35, P=0.007). This relation was strengthened after controlling for LDL-cholesterol concentration (r=-0.52, P<0.001).

CONCLUSIONS

In agreement with the view that small, dense LDL accelerates atherosclerosis, an inverse relationship was observed between LDL size and circulating in vivo oxidized LDL in Type 2 diabetic patients. Our results also suggest that in vitro susceptibility to oxidation is not a suitable surrogate measure for in vivo LDL oxidation.

Relationships between low-density lipoprotein particle size, plasma lipoproteins, and progression of coronary artery disease: the Diabetes Atherosclerosis Intervention Study (DAIS)

BACKGROUND

The Diabetes Atherosclerosis Intervention Study showed that treatment with fenofibrate decreases progression of coronary atherosclerosis in subjects with type 2 diabetes. We determined whether on-treatment plasma lipid concentrations and LDL particle size contribute to the favorable effect of fenofibrate on the progression of coronary artery disease (CAD).

METHODS AND RESULTS

A total of 418 subjects with type 2 diabetes were randomly assigned to 200 mg micronized fenofibrate daily or placebo. The mean follow-up time was 39.6 months. LDL peak particle diameter (LDL size) was determined by polyacrylamide gradient gel electrophoresis from 405 subjects at baseline and at the end of the study. Progression of CAD was measured with quantitative coronary angiography. LDL size increased significantly more in the fenofibrate group than in the placebo group (0.98+/-1.04 versus 0.32+/-0.92 nm, P<0.001). In the combined group, small LDL size was significantly associated with progression of CAD measured as the increase of percentage diameter stenosis (r=-0.16, P=0.002) and decreases in minimum (r=-0.11, P=0.030) and mean (r=-0.10, P=0.045) lumen diameter. High on-treatment LDL cholesterol, apolipoprotein B, and triglyceride concentrations were also associated with the progression of CAD. In regression analyses, small LDL size added to the effect of LDL cholesterol and apolipoprotein B on the progression of CAD. Similar associations were observed in the fenofibrate group, whereas in the placebo group, lipoprotein variables were not significantly correlated with the progression of CAD.

CONCLUSIONS

Changes in LDL size and plasma lipid levels account for part of the antiatherogenic effect of fenofibrate in type 2 diabetes.

Smallest LDL particles are most strongly related to coronary disease progression in men

OBJECTIVE

LDLs include particle subclasses that have different mobilities on polyacrylamide gradient gels: LDL-I (27.2 to 28.5 nm), LDL-IIa (26.5 to 27.2 nm), LDL-IIb (25.6 to 26.5 nm), LDL-IIIa (24.7 to 25.6 nm), LDL-IIIb (24.2 to 24.7 nm), LDL-IVa (23.3 to 24.2 nm), and LDL-IVb (22.0 to 23.3 nm in diameter). We hypothesized that the association between smaller LDL particles and coronary artery disease (CAD) risk might involve specific LDL subclasses.

METHODS AND RESULTS

Average 4-year onstudy lipoprotein measurements were compared with annualized rates of stenosis change from baseline to 4 years in 117 men with CAD. The percentages of total LDL and HDL occurring within individual subclasses were measured by gradient gel electrophoresis. Annual rate of stenosis change was related concordantly to onstudy averages of total cholesterol (P=0.04), triglycerides (P=0.05), VLDL mass (P=0.03), total/HDL cholesterol ratio (P=0.04), LDL-IVb (P=0.01), and HDL(3a) (P=0.02) and inversely to HDL(2)-mass (P=0.02) and HDL(2b) (P=0.03). The average annual rate in stenosis change was 6-fold more rapid in the fourth quartile of LDL-IVb (>or=5.2%) than in the first quartile (<2.5%, P=0.03). Stepwise multiple regression analysis showed that LDL-IVb was the single best predictor of stenosis change.

CONCLUSIONS

LDL-IVb was the single best lipoprotein predictor of increased stenosis, an unexpected result, given that LDL-IVb represents only a minor fraction of total LDL.

Plasma lipid composition and LDL oxidation

Low-density lipoprotein (LDL) oxidation in vivo depends on lipid composition and on plasma antioxidant status. The aim of our study was to investigate the relationship between plasma lipid composition and LDL oxidation and, in particular, to explore whether LDL-cholesterol/triglycerides ratio (LDL-C/TG) and LDL-cholesterol/high-density lipoprotein (HDL)-cholesterol ratio (LDL-C/HDL-C) can be used as predictive parameters of LDL oxidation in vivo. In 87 volunteers over a wide range of age plasma lipids and LDL oxidation were studied. Blood was collected after 12 h overnight fast. LDL oxidation was estimated by the level of conjugated diene (BDC) in the lipid fraction isolated from plasma after gradient ultra-centrifugation. The results were expressed as micromol/l (BDC/l) to evaluate the level of oxidized LDL, and as nmol of BDC for mg of LDL-cholesterol (BDC/LDL-C) for the evaluation of LDL oxidation degree. BDC/l correlated significantly with age, total and LDL-C, apolipoprotein B and TG, while BDC/LDL-C negatively correlated with total cholesterol, apolipoprotein B, LDL/TG and LDL/HDL ratios. Age of subjects significantly correlated with total and LDL-C and apolipoprotein B. TG have a significant inverse correlation with HDL-C. Our results support the hypothesis that among the several factors involved in LDL oxidation the most important determinants are LDL/TG. Plasma triglycerides appear to be very important even when circulating cholesterol levels are within normal limits. Moreover, we found that the LDL/HDL ratio is also very important with regard to the putative protective role of HDL against LDL oxidation in vivo. In conclusion, plasma lipid parameters must be evaluated not only for their absolute values but also for their mutual ratios as expression of plasma lipid homeostasis. Both LDL/TG and LDL/HDL ratios can be used as predictive parameters of in vivo LDL oxidation.

Assay methods of modified lipoproteins in plasma

Modified lipoproteins, especially oxidatively modified low-density lipoprotein (Ox-LDL), are present in the plasma of patients with atherosclerosis and related diseases. The modification of LDL is believed to play an important role in the development of atherosclerosis. Thus, measurement of plasma Ox-LDL is essential not only for investigating its relevance to atherosclerotic diseases, but also for diagnosis. Chromatographic methods are effective for indirectly measuring the oxidatively modified state of LDL or directly measuring the modified LDL. Indirect determination can be done by estimating the LDL subfraction, LDL particle size, oxidized amino acids in apolipoprotein B, lipid hydroperoxide or F(2)-isoprostane in LDL. Direct determination of the modified LDL in plasma can be done with chromatographic methods such as anion-exchange chromatography and size-exclusion chromatography. Other methods for estimating the modified state of LDL include electromigration methods such as agarose gel, polyacrylamide gradient gel and capillary electrophoresis. Recently, enzyme-linked immunosorbent assay methods of malondialdehyde (MDA)-LDL and autoantibodies against Ox-LDL have been developed to assess Ox-LDL in plasma. This review article summarizes the detection and assay methods of modified lipoproteins in plasma.

Significance of small dense low-density lipoproteins and other risk factors in patients with various types of coronary heart disease

BACKGROUND

It remains unclear how closely the small dense low-density lipoprotein (LDL) (diameter < or =25.5 nm) is associated with various types of coronary heart disease (CHD) in Japanese patients, an ethnic group with lower serum cholesterol levels and less massive obesity compared with Western populations.

METHODS AND RESULTS

We measured mean LDL particle diameter by gradient gel electrophoresis in 571 patients with CHD and in 263 healthy subjects who served as control patients. Patients with CHD were classified into acute coronary syndrome (ACS), stable CHD and vasospastic angina. High-density lipoprotein cholesterol and apolipoprotein-A1 and -B were significantly different between patients with CHD and controls. LDL size in patients with CHD was markedly smaller than that in controls in both men and women (25.5 +/- 0.7 vs 25.9 +/- 0.4 and 25.7 +/- 0.7 vs 26.0 +/- 0.5 nm, respectively). LDL cholesterol was significantly higher in patients with ACS than in other groups. Plasma levels of high-density lipoprotein cholesterol decreased as the number of diseased vessels or angiographic coronary severity evaluated by Gensini score increased, but the LDL size was comparable irrespective of the type of CHD and the extent and severity of the lesions. Multiple logistic regression analysis revealed that small dense LDL was independently associated with the incidence of CHD in both sexes (odds ratio [OR] 3.5, 95% CI 2.1-5.7, and OR 2.9, 95% CI 1.5-5.6, P <.005).

CONCLUSION

Our study suggests that the small dense LDL is strongly associated with various types of CHD, independent of traditional and nontraditional coronary risk factors, but is not related to the severity and extent of the coronary lesions.

Endothelial vasodilatory function is predicted by circulating apolipoprotein B and HDL in healthy humans

Endothelium-dependent vasodilation (EDV), LDL particle size, and antibodies against oxidized LDL (oxLDLab) have been shown to be related to the development of atherosclerosis and cardiovascular disease. In this study, we investigated whether LDL particle size, oxLDLab, apolipoproteins, and lipoproteins are related to endothelial vasodilatory function in a population sample of 58 apparently healthy subjects aged 20 to 69 yr. EDV and endothelium-independent vasodilation (EIDV) were studied in the forearm during local administration of methacholine chloride (2 and 4 microg/min) or sodium nitroprusside (5 and 10 microg/min). Forearm blood flow was determined with venous occlusion plethysmography. In multiple stepwise regression analyses, neither oxLDLab nor small LDL particles were significantly predictive of endothelial vasodilatory function. Instead, a high level of apolipoprotein B (apoB) was an independent predictor of both attenuated EDV and EIDV (r = -0.43, P < 0.01, and r = -0.34, P < 0.05, respectively). HDL cholesterol, on the other hand, was the only lipid variable that was significantly related to the EDV to EIDV ratio, an index of endothelial vasodilatory function (r = 0.35, P < 0.01). The inverse associations between apoB and both EDV and EIDV indicate that apoB might be an early marker of structural vascular changes in healthy subjects, whereas HDL seems to be more specifically related to endothelial vasodilatory function.

Clinical relevance of the biochemical, metabolic, and genetic factors that influence low-density lipoprotein heterogeneity

Traditional risk factors for coronary artery disease (CAD) predict about 50% of the risk of developing CAD. The Adult Treatment Panel (ATP) III has defined emerging risk factors for CAD, including small, dense low-density lipoprotein (LDL). Small, dense LDL is often accompanied by increased triglycerides (TGs) and low high-density lipoprotein (HDL). An increased number of small, dense LDL particles is often missed when the LDL cholesterol level is normal or borderline elevated. Small, dense LDL particles are present in families with premature CAD and hyperapobetalipoproteinemia, familial combined hyperlipidemia, LDL subclass pattern B, familial dyslipidemic hypertension, and syndrome X. The metabolic syndrome, as defined by ATP III, incorporates a number of the components of these syndromes, including insulin resistance and intra-abdominal fat. Subclinical inflammation and elevated procoagulants also appear to be part of this atherogenic syndrome. Overproduction of very low-density lipoproteins (VLDLs) by the liver and increased secretion of large, apolipoprotein (apo) B-100-containing VLDL is the primary metabolic characteristic of most of these patients. The TG in VLDL is hydrolyzed by lipoprotein lipase (LPL) which produces intermediate-density lipoprotein. The TG in intermediate-density lipoprotein is hydrolyzed further, resulting in the generation of LDL. The cholesterol esters in LDL are exchanged for TG in VLDL by the cholesterol ester tranfer proteins, followed by hydrolysis of TG in LDL by hepatic lipase which produces small, dense LDL. Cholesterol ester transfer protein mediates a similar lipid exchange between VLDL and HDL, producing a cholesterol ester-poor HDL. In adipocytes, reduced fatty acid trapping and retention by adipose tissue may result from a primary defect in the incorporation of free fatty acids into TGs. Alternatively, insulin resistance may promote reduced retention of free fatty acids by adipocytes. Both these abnormalities lead to increased levels of free fatty acids in plasma, increased flux of free fatty acids back to the liver, enhanced production of TGs, decreased proteolysis of apo B-100, and increased VLDL production. Decreased removal of postprandial TGs often accompanies these metabolic abnormalities. Genes regulating the expression of the major players in this metabolic cascade, such as LPL, cholesterol ester transfer protein, and hepatic lipase, can modulate the expression of small, dense LDL but these are not the major defects. New candidates for major gene effects have been identified on chromosome 1. Regardless of their fundamental causes, small, dense LDL (compared with normal LDL) particles have a prolonged residence time in plasma, are more susceptible to oxidation because of decreased interaction with the LDL receptor, and enter the arterial wall more easily, where they are retained more readily. Small, dense LDL promotes endothelial dysfunction and enhanced production of procoagulants by endothelial cells. Both in animal models of atherosclerosis and in most human epidemiologic studies and clinical trials, small, dense LDL (particularly when present in increased numbers) appears more atherogenic than normal LDL. Treatment of patients with small, dense LDL particles (particularly when accompanied by low HDL and hypertriglyceridemia) often requires the use of combined lipid-altering drugs to decrease the number of particles and to convert them to larger, more buoyant LDL. The next critical step in further reduction of CAD will be the correct diagnosis and treatment of patients with small, dense LDL and the dyslipidemia that accompanies it.

Association between the --514 C-->T polymorphism of the hepatic lipase gene promoter and unstable carotid plaque in patients with severe carotid artery stenosis

OBJECTIVE

We investigated the potential association between -514 C-->T polymorphism in the promoter of the hepatic lipase gene (LIPC) and the prevalence of inflammatory cells in the plaque of patients with severe carotid artery stenosis.

BACKGROUND

This common LIPC polymorphism has been related to the presence of an atherogenic lipoprotein pattern.

METHODS

We studied 68 consecutive patients undergoing carotid endarterectomy. The LIPC genotype was determined by polymerase chain reaction. Endarterectomy specimens were examined by immunocytochemistry using monoclonal antibodies for smooth muscle cells, macrophages, or lymphocytes.

RESULTS

In 50 of 68 patients who had evidence of previous ipsilateral ischemic events, 36 (72%) were carriers of the CC genotype, whereas only 14 (28%) were carriers of the CT/TT genotype (p = 0.002). Among the 18 patients without evidence of events, the two genotypes were equally distributed (9 vs. 9). The low-density lipoprotein (LDL) particles were denser in CC than in CT/TT genotype carriers (flotation rate: 0.315 +/- 0.025 vs. 0.356 +/- 0.019, p < 0.0005). The CC genotype was associated with an abundance of macrophages (6.7 +/- 3.5 vs. 2.1 +/- 2.1 cells/area unit in the CT/TT group, p < 0.0005) and a reduced number of smooth muscle cells (6.9 +/- 6.2 vs. 14.5 +/- 6.4 in the CT/TT group, p < 0.0005) in the plaque. An inverse relationship was found between LDL buoyancy and the number of macrophages in the plaque (r = -0.639, p < 0.0005).

CONCLUSION

We provide evidence, for the first time, that LIPC promoter -514 C-->T polymorphism, by modulating LDL density, significantly affects the number of macrophages in the plaque and possibly affects the occurrence of cerebrovascular events in patients with carotid artery stenosis.

Metabolic origins and clinical significance of LDL heterogeneity

LDLs in humans comprise multiple distinct subspecies that differ in their metabolic behavior and pathologic roles. Metabolic turnover studies suggest that this heterogeneity results from multiple pathways, including catabolism of different VLDL and IDL precursors, metabolic remodeling, and direct production. A common lipoprotein profile designated atherogenic lipoprotein phenotype is characterized by a predominance of small dense LDL particles. Multiple features of this phenotype, including increased levels of triglyceride rich lipoprotein remnants and IDLs, reduced levels of HDL and an association with insulin resistance, contribute to increased risk for coronary heart disease compared with individuals with a predominance of larger LDL. Increased atherogenic potential of small dense LDL is suggested by greater propensity for transport into the subendothelial space, increased binding to arterial proteoglycans, and susceptibility to oxidative modification. Large LDL particles also can be associated with increased coronary disease risk, particularly in the setting of normal or low triglyceride levels. Like small LDL, large LDL exhibits reduced LDL receptor affinity compared with intermediate sized LDL. Future delineation of the determinants of heterogeneity of LDL and other apoB-containing lipoproteins may contribute to improved identification and management of patients at high risk for atherosclerotic disease.

Relations of lipoprotein subclass levels and low-density lipoprotein size to progression of coronary artery disease in the Pravastatin Limitation of Atherosclerosis in the Coronary Arteries (PLAC-I) trial

Lipoprotein subclass measurements may enhance the prediction of coronary artery disease (CAD) risk, but clinical application of such information has been hindered by the relatively laborious and time-consuming nature of laboratory measurement methods. In this study, lipoprotein subclass analyses were performed on frozen plasma samples from 241 participants in the Pravastatin Limitation of Atherosclerosis in the Coronary arteries Trial using an automated nuclear magnetic resonance technique. The objective was to determine if levels of these subclasses provided additional information on the progression of CAD, based on the change in the minimum lumen diameter, over a 3-year period. After adjustment for race, sex, age, treatment group, baseline lumen diameter, and chemically measured levels of triglycerides, low-density lipoprotein (LDL) cholesterol, and high-density lipoprotein (HDL) cholesterol, on-trial predictors (p <0.05) of progression included an elevated LDL particle number, and levels of small LDL and small HDL. Within treatment groups, CAD progression was most strongly related to the LDL particle number (placebo) and levels of small HDL (pravastatin). In logistic regression models that adjusted for chemically determined lipid levels and other covariates, a small LDL level > or = 30 mg/dl (median) was associated with a ninefold increased risk of CAD progression (p <0.01) in the placebo group. These results indicate that levels of various lipoprotein subclasses may provide useful information on CAD risk even if levels of traditional risk factors are known.

DIETARY ANDGENETICEFFECTS ONLOW-DENSITYLIPOPROTEINHETEROGENEITY

We have tested whether differences in distribution and dietary responsiveness of low-density lipoprotein (LDL) subclasses contribute to the variability in the magnitude of LDL-cholesterol reduction induced by diets low in total and saturated fat and high in carbohydrate. Our studies have focused on a common, genetically influenced metabolic profile, characterized by a predominance of small, dense LDL particles (subclass pattern B), that is associated with a two- to threefold increase in risk for coronary artery disease. We have found that healthy normolipidemic individuals with this trait show a greater reduction in LDL cholesterol and particle number in response to low-fat, high-carbohydrate diets than do unaffected individuals (subclass pattern A). Moreover, such diets result in reduced LDL particle size, with induction of pattern B in a substantial proportion of pattern A men. Recent studies have indicated that this response is under genetic influence. Future identification of the specific genes involved may lead to improved targeting of dietary therapies aimed at reducing cardiovascular disease risk.

Hepatic lipase as a focal point for the development and treatment of coronary artery disease

Recent epidemiological evidence suggests that although lowering low-density lipoprotein (LDL) cholesterol is important in decreasing cardiovascular disease morbidity and mortality, it accounts only for part of the coronary artery disease (CAD) improvement with lipid-lowering therapy. In the last decade, it has become evident that the atherogenicity of LDL particles is associated not only with their plasma levels, but also with their size and density. The presence of small, dense LDL particles is associated with a three fold increase in CAD risk. Hepatic lipase (HL), a key enzyme in the formation of small, dense LDL particles, modulates their phospholipid and triglyceride contents. The higher the HL activity, the smaller, denser, and more atherogenic the resulting lipoprotein particle. It is, therefore, plausible to hypothesize that at least part of the CAD benefits observed in the recent CAD-prevention pharmacological trials, which are not accounted for by the decrease in LDL-C (LDL-cholesterol), might be explained by a pharmacological effect on LDL size and density, possibly mediated by changes in hepatic lipase activity. By studying patients with dyslipidemia and CAD, we have been able to provide strong evidence that regression of coronary atherosclerosis results from at least two independent effects of lipid-lowering therapy on lipoprotein metabolism: the well known one that leads to changes in LDL-C and apo B levels, and a new pathway of HL-mediated improvement in LDL buoyancy. Finally, HL activity and LDL density appear to be significantly affected by the presence of a common C-->T substitution at position -514 with respect to the transcription start site of the HL gene, raising the possibility that the -514 C-->T polymorphism may significantly contribute to differences in individual CAD response to lipid-lowering treatment, as seen in the recent major primary and secondary CAD-prevention clinical trials.

Hypertriglyceridemia: associated risks and effect of drug treatment

Accumulating evidence indicates that hypertriglyceridemia (HTG) is a risk factor for cardiovascular disease. This increased risk is probably substantially mediated through the metabolic interrelationships between serum triglyceride (TG) levels and other risk factors, such as the atherogenic lipid profile (low high density lipoprotein-cholesterol levels and elevated small dense low density lipoprotein levels), insulin resistance, a prothrombotic propensity and low grade systemic inflammation. TG-lowering strategy in patients with HTG encompasses dietary modification and pharmacological agents, such as fibric acid derivatives, fish-oil and hydroxymethylglutaryl coenzyme A reductase inhibitors, which have, besides their known effects on the atherogenic lipid profile, beneficial effects on other determinants of cardiovascular disease. However, in spite of data from trials investigating fibric acid derivative-induced reduction in coronary events in patients with distinct types of hyperlipidemia, no specific trials have been performed that investigated this risk reduction in patients with HTG, in whom other cardiovascular risk factors are clustered as well. Small-scale studies on determinants of cardiovascular disease in patients with HTG and post-hoc analyses of the Helsinki Heart, Veterans Affairs High-Density Lipoprotein Cholesterol Intervention Trial and Bezafibrate Infarction Prevention trials in patients with high serum TG levels suggest a drug-induced reduction in cardiovascular events. However, a specific trial should be conducted to investigate the effects of lipid-lowering therapy on clinical end-points in patients with HTG of defined types.

Phospholipase A2 and small, dense low-density lipoprotein

High levels of small, dense LDL in plasma are associated with increased risk for cardiovascular disease. There are some biochemical characteristics that may render small, dense LDL particles more atherogenic than larger, buoyant LDL particles. First, small, dense LDL particles contain less phospholipids and unesterified cholesterol in their surface monolayer than do large, buoyant LDL particles. This difference in lipid content appears to induce changes in the conformation of apolipoprotein B-100, leading to more exposure of proteoglycan-binding regions. This may be one reason for the high-affinity binding of small, dense LDL to arterial proteoglycans. Reduction of the phospholipid content in the surface monolayer LDL by treatment with secretory phospholipase A2 (sPLA2) forms small, dense LDL with an enhanced tendency to interact with proteoglycans. Circulating levels of sPLA2-IIA appears to be an independent risk factor for coronary artery disease and a predictor of cardiovascular events. In addition, in-vivo studies support the hypothesis that sPLA2 proteins contribute to atherogenesis and its clinical consequences. These data suggest that modification of LDL by sPLA2 in the arterial tissue or in plasma may be a mechanism for the generation of atherogenic lipoprotein particles in vivo, with a high tendency to be entrapped in the arterial extracellular matrix.

The metabolic syndrome, LDL particle size, and atherosclerosis: the Atherosclerosis and Insulin Resistance (AIR) study

An operative definition of the metabolic syndrome has been suggested by a working group associated with the World Health Organization in 1998. The aim of this study was to examine whether small, low density lipoprotein (LDL) particle size was associated with the metabolic syndrome and with subclinical atherosclerosis as measured by ultrasound in the carotid and femoral arteries. The study was performed in a population-based sample of clinically healthy men (N=391), all 58 years old and not undergoing any treatment with cardiovascular drugs. Exclusion criteria were cardiovascular or other clinically overt diseases or continuous medication with cardiovascular drugs. The results showed that subjects characterized by the metabolic syndrome (n=62) had a thicker mean intima-media complex (IMT) in both the carotid and femoral arteries (0.86 versus 0.77 mm, P:<0.001, and 1.03 versus 1. 00 mm, P:=0.022, respectively) and also lower mean values for LDL particle size (25.78 versus 26.80 nm, respectively, P:<0.001) compared with subjects with no risk factors (n=77). The group with the metabolic syndrome (n=62) also had higher mean values for serum cholesterol and heart rate. In the whole study group (N=391), there were significant but weak negative relationships between small LDL particle size, increasing IMT, and increasing cross-sectional intima-media area of the carotid and femoral arteries and also negative relationships between LDL particle size and plaque occurrence and size in the carotid and femoral arteries. In summary, this is the first large-scale study to demonstrate a relationship between the clustering of risk factors that constitute the metabolic syndrome and a small LDL particle size pattern and the occurrence of preclinical atherosclerosis in the carotid and femoral arteries, as assessed by the ultrasound technique, in healthy 58-year-old men recruited from the general population.

Preventing, stopping, or reversing coronary artery disease--triglyceride-rich lipoproteins and associated lipoprotein and metabolic abnormalities: the need for recognition and treatment

A substantial number of treated patients with or at high risk for coronary artery disease continue to have fatal and nonfatal coronary artery events in spite of significant reduction of elevated levels of low-density lipoprotein cholesterol. Other lipoprotein abnormalities besides an elevated level of low-density lipoprotein cholesterol contribute to risk of coronary artery disease and coronary artery events, and the predominant abnormalities that appear to explain much of this continued risk are an elevated serum triglyceride level and a low level of high-density lipoprotein cholesterol. Most patients with coronary artery disease have a mixed dyslipidemia with hypertriglyceridemia, which is associated and metabolically intertwined with other atherogenic risk factors, including the presence of triglyceride-rich lipoprotein remnants, low levels of high-density lipoprotein cholesterol, small, dense, low-density lipoprotein particles, postprandial hyperlipidemia, and a prothrombotic state. Aggressive treatment of these patients needs to focus on these other lipoprotein abnormalities as much as on low-density lipoprotein cholesterol. Combination drug therapy will usually be required. Reliable assessment of risk of coronary artery disease from lipoprotein measurements and response to therapy requires inclusion of all atherogenic lipoproteins in laboratory measurements and treatment protocols. At present this may be best accomplished by use of non-high-density lipoprotein cholesterol (total cholesterol minus high-density lipoprotein cholesterol) calculated from standard laboratory lipoprotein values. Ultimately, a more comprehensive assessment of coronary artery disease risk and appropriate therapy may include measurement of lipoprotein subclass distribution including determination of low-density lipoprotein particle concentration and sizes of the various lipoprotein particles.

LDL particle size in relation to intima-media thickness and plaque occurrence in the carotid and femoral arteries in patients with hypercholesterolaemia

OBJECTIVES

Small LDL particle size has been shown to be associated with coronary artery disease. However, no previous study has been performed relating LDL particle size to ultrasound measurement of atherosclerosis in hypercholesterolaemic subjects. The main aims of the present study were therefore: (i) to investigate the relationship between intima-media thickness (IMT) of the carotid bulb and LDL particle size in patients with primary hypercholesterolaemia (n = 102) and in matched normocholesterolaemic controls (n = 102); and (ii) to investigate the relationship between plaque occurrence in the carotid and femoral arteries and LDL particle size. LDL particle size was determined by subjecting serum to 2-16% polyacrylamide gradient gel electrophoresis.

RESULTS

The results showed that patients with primary hypercholesterolaemia had smaller LDL peak particle size compared with healthy control subjects (P < 0.001 for men, P = 0.006 for women). However, the difference in LDL peak particle size between patients with hypercholesterolaemia and controls disappeared when adjusting for serum triglycerides. There was no association between increasing IMT of the carotid bulb and decreasing LDL peak particle size or between the occurrence of moderate to large plaques in the carotid and femoral arteries and small LDL peak particle size. However, metabolic variables such as serum triglycerides, HDL, blood glucose, body mass index, and also systolic blood pressure were associated with LDL peak particle size.

CONCLUSION

The lack of an association between atherosclerosis, as measured by ultrasound, and small LDL particle size in patients with hypercholesterolaemia implies that other risk factors, such as hypercholesterolaemia and high blood pressure, may overshadow any importance of small LDL particle size.

High density lipoprotein oxidation: in vitro susceptibility and potential in vivo consequences

Elevated levels of plasma high density lipoprotein (HDL) are strongly predictive of protection against atherosclerotic vascular disease. HDL particles likely have several beneficial actions in vivo, including the initiation of reverse cholesterol transport. The apparent importance of oxidative modification of low density lipoprotein in atherogenesis raises the question of how oxidative modification of HDL might affect its cardioprotective actions. HDL is readily oxidized using numerous models of lipoprotein oxidation. In vitro evidence suggests oxidation might impair some protective actions, but actually enhance other mechanisms induced by HDL that prevent the accumulation of cholesterol in the artery wall. This article reviews the current literature concerning the relative oxidizability of HDL, the structural changes induced in HDL by oxidation in vitro, and the potential consequences of oxidative modification on the protective actions of HDL in vivo.

Phospholipase A(2) modification of low density lipoproteins forms small high density particles with increased affinity for proteoglycans and glycosaminoglycans

The presence of a lipoprotein profile with abundance of small, dense low density lipoproteins (LDL), low levels of high density lipoproteins (HDL), and elevated levels of triglyceride-rich very low density lipoproteins is associated with an increased risk for coronary heart disease. The atherogenicity of small, dense LDL is believed to be one of the main reasons for this association. This particle contains less phospholipids (PL) and unesterified cholesterol than large LDL, and the apoB-100 appears to occupy a more extensive area at its surface. Although there are experiments that suggest a metabolic pathway leading to the overproduction of small, dense LDL, no clear molecular model exists to explain its association with atherogenesis. A current hypothesis is that small, dense LDL, because of its higher affinity for proteoglycans, is entrapped in the intima extracellular matrix and is more susceptible to oxidative modifications than large LDL. Here we describe how a specific reduction of approximately 50% of the PL of a normal buoyant LDL by immobilized phospholipase A(2) (PLA(2)) (EC 3.1.1.4) produces smaller and denser particles without inducing significant lipoprotein aggregation (<5%). These smaller LDL particles display a higher tendency to form nonsoluble complexes with proteoglycans and glycosaminoglycans than the parent LDL. Binding parameters of LDL and glycosaminoglycans and proteoglycans produced by human arterial smooth muscle cells were measured at near to physiological conditions. The PLA(2)-modified LDL has about 2 times higher affinity for the sulfated polysaccharides than control LDL. In addition, incubation of human plasma in the presence of PLA(2) generated smaller LDL and HDL particles compared with the control plasma incubated without PLA(2). These in vitro results indicate that the reduction of surface PL characteristic of small, dense LDL subfractions, besides contributing to its small size and density, may enhance its tendency to be retained by proteoglycans.

Hypertriglyceridemia: changes in the plasma lipoproteins associated with an increased risk of cardiovascular disease

There is a growing body of evidence from epidemiologic, clinical, and laboratory data that indicates that elevated triglyceride levels are an independent risk factor for cardiovascular disease. Identification and quantification of atherogenic lipoproteins in patients with hypertriglyceridemia are important steps in the prevention of cardiovascular disease. Increased levels of apoC-III, apoC-I, or apoA-II on the apoB-containing lipoproteins may alter lipoprotein metabolism and result in the accumulation of atherogenic remnants. Hypertriglyceridemic patients at risk for cardiovascular disease often develop a lipoprotein profile characterized by elevated triglyceride, dense LDL, and low HDL cholesterol. Understanding that each of these factors contributes separately to the patient's risk of cardiovascular disease can help physicians provide patients with more effective risk-reduction programs for cardiovascular disease.

Evidence for a new pathophysiological mechanism for coronary artery disease regression: hepatic lipase-mediated changes in LDL density

BACKGROUND

Small, dense LDL particles are associated with coronary artery disease (CAD) and predict angiographic changes in response to lipid-lowering therapy. Intensive lipid-lowering therapy in the Familial Atherosclerosis Treatment Study (FATS) resulted in significant improvement in CAD. This study examines the relationship among LDL density, hepatic lipase (HL), and CAD progression, identifying a new biological mechanism for the favorable effects of lipid-altering therapy.

METHODS AND RESULTS

Eighty-eight of the subjects in FATS with documented coronary disease, apolipoprotein B levels >/=125 mg/dL, and family history of CAD were selected for this study. They were randomly assigned to receive lovastatin (40 mg/d) and colestipol (30 g/d), niacin (4 g/d) and colestipol, or conventional therapy with placebo alone or with colestipol in those with elevated LDL cholesterol levels. Plasma hepatic lipase (HL), lipoprotein lipase, and LDL density were measured when subjects were and were not receiving lipid-lowering therapy. LDL buoyancy increased with lovastatin-colestipol therapy (7.7%; P<0.01) and niacin-colestipol therapy (10.3%; P<0.01), whereas HL decreased in both groups (-14% [P<0.01] and -17% [P<0.01] with lovastatin-colestipol and niacin-colestipol, respectively). Changes in LDL buoyancy and HL activity were associated with changes in disease severity (P<0.001). In a multivariate analysis, an increase in LDL buoyancy was most strongly associated with CAD regression, accounting for 37% of the variance of change in coronary stenosis (P<0.01), followed by reduction in apolipoprotein Bl (5% of variance; P<0.05).

CONCLUSIONS

These studies support the hypothesis that therapy-associated changes in HL alter LDL density, which favorably influences CAD progression. This is a new and potentially clinically relevant mechanism linking lipid-altering therapy to CAD improvement.

Direct assessment of lipoprotein outflow from in vivo-labeled arterial tissue as determined in an in vitro perfusion system

The rate of cholesterol deposition during the atherosclerotic process is determined by the balance between the inflow and outflow of plasma lipoproteins in the arterial wall. Whereas the rate of inflow may be measured directly, the rate of outflow has most often been calculated indirectly from lipoprotein uptake by using the 2-compartment model. One objection against such calculations is that lipoprotein binding is not being considered. In the present study 2 different protocols were used to obtain a direct measure of the outflow of lipoproteins from atherosclerotic rabbit aortas. Thus, 3 rabbits with experimental atherosclerosis were given 125I-LDL intravenously and 3 were given [14C]cholesterol perorally. Twenty-four hours later the aortas were removed and the outflow of label was monitored during in vitro perfusion. Despite the different protocols, our results were consistent and indicated that fractional loss relative to whole tissue was approximately 0.01 pool/h, which is 1 order of magnitude lower than current estimates based on the 2-compartment model (0.04 to 0.4 pool/h). Furthermore, whereas as much as 2/3 to 3/4 of the tracer that had entered the arterial wall was effectively trapped, the remainder equilibrated at a faster rate (0.06 pool/h). In conclusion, it seems that tissue binding constitutes a prominent and possibly underrated mechanism of lipoprotein deposition, at least in the atherosclerotic rabbit aorta. Furthermore, this means that current estimates of lipoprotein exchange parameters based on the 2-compartment model (eg, fractional loss) may rest on invalid assumptions and should be regarded with caution.

Association of apo B lipoproteins with arterial proteoglycans: pathological significance and molecular basis

Retention of apo B-100 lipoproteins, low density lipoprotein (LDL) and probably lipoprotein(a), Lp(a), by intima proteoglycans (PGs) appears to increase the residence time needed for their structural, hydrolytic and oxidative modifications. If the rate of LDL entry exceeds the tissue capacity to eliminate the modified products, this process may be a contributor to atherogenesis and lesion advancement. LDL binds to PGs of the intima, by association of specific positive segments of the apo B-100 with the negatively-charged glycosaminoglycans (GAGs) made of chondroitin sulfate (CS), dermatan sulfate (DS) and probably heparan sulfate (HS). Small, dense LDL has a higher affinity for CS-PGs than large buoyant particles, probably because they expose more of the segments binding the GAGs than larger LDL. PGs cause irreversible structural alterations of LDL that potentiate hydrolytic and oxidative modifications. These alterations also increase LDL uptake by macrophages and smooth muscle cells. These in vitro data suggest that part of the atherogenicity of LDL may depend on its tendency to form complexes with arterial PGs in vivo. Ex vivo results support this hypothesis. Subjects with coronary heart disease have LDL with significantly higher affinity for arterial PGs. This is also a characteristic of subjects with the atherogenic lipoprotein phenotype, with high levels of small, dense LDL. The LDL-PG affinity, however can be modified by dietary or pharmacological interventions that change the composition and size of LDL. Lesion-prone intima contain PGs with a high affinity for LDL. Increased LDL entrapment at these sites may be a key step in a cyclic atherogenic process.

Triglycerides and atherogenic lipoproteins: rationale for lipid management

Epidemiologic and clinical studies have demonstrated a relation between plasma triglyceride levels and risk of coronary artery disease and an amplification of risk with combined elevations of triglyceride and low-density lipoprotein (LDL) cholesterol. In patients with coronary disease, angiographic progression and clinical events have been correlated with concentrations of smaller very-low-density lipoproteins (VLDL) and intermediate-density lipoproteins (IDL), consistent with evidence for enhanced atherogenicity of lipolytic products of triglyceride-rich lipoprotein metabolism, including postprandial lipoproteins. IDL levels also have been shown to be strongly and independently predictive of progression of carotid artery intimal-medial thickness, a measure of early atherogenesis that is related to coronary disease risk. Although there is evidence that these triglyceride-rich lipoprotein species may have direct atherogenic effects, other lipoprotein changes associated with altered triglyceride metabolism may be of particular importance in the development of coronary artery disease. These include reductions in high-density lipoprotein (HDL) and increases in small, dense LDL particles (LDL subclass pattern B). Because of the strong interrelations among elevated triglyceride, reduced HDL, and small dense LDL, it is difficult to use statistical techniques to determine the independent contributions of these traits to coronary disease risk. Based on their biologic properties, it is likely that each are involved in multiple steps of the disease process. Moreover, this cluster of lipoprotein changes is associated with other conditions that can promote vascular disease, including increases in coagulation factors and reduced insulin sensitivity. Analyses from intervention trials in patients with coronary disease have indicated that measurement of plasma triglyceride and LDL particle distributions can be of value in predicting the benefits of specific lipid-altering therapies on disease progression.

Effects of chylomicrons and chylomicron remnants on endothelium-dependent relaxation of rat aorta

The effects of chylomicrons and chylomicron remnants on endothelium-dependent relaxation of rat aorta were studied in vitro. Chylomicrons and chylomicron remnants were prepared in vivo. Aortic rings were incubated with the lipoproteins for 45 min before the vessels were constricted with phenylephrine and concentration relaxation response curves constructed to carbachol, ATP, A23187 and S-nitroso-N-acetylpenicillamine. Maximum % relaxations to carbachol were significantly reduced by both chylomicrons and chylomicron remnants but responses to ATP and S-nitroso-N-acetylpenicillamine were unaffected. In addition, chylomicrons significantly inhibited A23187-induced relaxation, causing an increase in the EC50 value. Chylomicron remnants cause selective inhibition of carbachol-induced relaxation suggesting an action at the receptor or G protein-coupled component of the receptor-mediated activation of the L-arginine-nitric oxide pathway. Chylomicrons appear to be less selective in their inhibition of the endothelium-dependent relaxation. This study demonstrates that lipoprotein particles of dietary origin may cause endothelial cell dysfunction.