Heterogeneity of apolipoprotein A-I turnover in subjects with reduced concentrations of plasma high density lipoprotein cholesterol

Dyslipidaemia in type 2 diabetes: epidemiology and biochemistry

Lipid abnormalities in type 2 diabetes are characterised by high triglyceride concentrations, low high density lipoprotein-cholesterol concentrations, and normal total and low density lipoprotein-cholesterol (LDL-c) concentrations. LDL particles, however, are small and dense. Epidemiological evidence for these lipid abnormalities, and for the associations between lipid abnormalities and the increased risk of cardiovascular disease in type 2 diabetes, are presented. Most, of the lipid abnormalities in type 2 diabetes can be explained by reduced action of insulin at the tissue level. The biochemical and metabolic mechanisms underlying the lipid abnormalities are discussed.

Cholesteryl Ester Transfer Protein Inhibition With Anacetrapib Decreases Fractional Clearance Rates of High-Density Lipoprotein Apolipoprotein A-I and Plasma Cholesteryl Ester Transfer Protein

OBJECTIVE

Anacetrapib (ANA), an inhibitor of cholesteryl ester transfer protein (CETP) activity, increases plasma concentrations of high-density lipoprotein cholesterol (HDL-C), apolipoprotein A-I (apoA)-I, apoA-II, and CETP. The mechanisms responsible for these treatment-related increases in apolipoproteins and plasma CETP are unknown. We performed a randomized, placebo (PBO)-controlled, double-blind, fixed-sequence study to examine the effects of ANA on the metabolism of HDL apoA-I and apoA-II and plasma CETP.

APPROACH AND RESULTS

Twenty-nine participants received atorvastatin (ATV) 20 mg/d plus PBO for 4 weeks, followed by ATV plus ANA 100 mg/d for 8 weeks (ATV-ANA). Ten participants received double PBO for 4 weeks followed by PBO plus ANA for 8 weeks (PBO-ANA). At the end of each treatment, we examined the kinetics of HDL apoA-I, HDL apoA-II, and plasma CETP after D3-leucine administration as well as 2D gel analysis of HDL subspecies. In the combined ATV-ANA and PBO-ANA groups, ANA treatment increased plasma HDL-C (63.0%; P<0.001) and apoA-I levels (29.5%; P<0.001). These increases were associated with reductions in HDL apoA-I fractional clearance rate (18.2%; P=0.002) without changes in production rate. Although the apoA-II levels increased by 12.6% (P<0.001), we could not discern significant changes in either apoA-II fractional clearance rate or production rate. CETP levels increased 102% (P<0.001) on ANA because of a significant reduction in the fractional clearance rate of CETP (57.6%, P<0.001) with no change in CETP production rate.

CONCLUSIONS

ANA treatment increases HDL apoA-I and CETP levels by decreasing the fractional clearance rate of each protein.

Elimination of endoplasmic reticulum stress and cardiovascular, type 2 diabetic, and other metabolic diseases

Multiple factors including unhealthy living habits influence the life-maintaining functions of the endoplasmic reticulum (ER) and induce ER stress and metabolic abnormalities. The ER responds to the disturbances by activating mechanisms that increase the capacity to eliminate ER stress. This article elucidates the effects of ER activation that eliminates both ER stress and associated cardiovascular, type 2 diabetic (DM2), and other metabolic diseases. ER-activating compounds eliminate ER stress by lowering elevated cholesterol, regress atherosclerosis, decrease cardiovascular mortality, reduce blood glucose and insulin, and, together with the normalization of glucose-insulin homeostasis, remove insulin resistance, pancreatic β-cell failure, and DM2. A deficient cytochrome P450 activity in hepatic ER leads to cholesterol accumulation that induces stress and xanthoma formation, whereas P450-activating therapy up-regulates apolipoprotein AI and LDLR genes, down-regulates apolipoprotein B gene, and produces an antiatherogenic plasma lipoprotein profile. The ER activation reduces the stress also by eliminating hepatic fat and converting saturated fatty acids (FAs) to unsaturated FAs. Cognitive processes require gene expression modification, and preclinical studies indicate that ER-activating therapy improves cognition. Promotion of healthy lifestyle choices and indicated therapies are key factors in the prevention and elimination of ER stress and associated global health problems.

Unacylated Ghrelin is associated with the isolated low HDL-cholesterol obese phenotype independently of insulin resistance and CRP level

BACKGROUND

Low plasma high-density lipoprotein-cholesterol (HDL-c) level is commonly present in obesity and represents an independent cardiovascular risk factor. However, obese patients are a very heterogeneous population and the factors and mechanisms that contribute to low HDL-c remain unclear. The aim of this study was to investigate the association between plasma HDL-c levels and plasma hormonal profiles (insulin, adiponectin, resistin, leptin and ghrelin) in subsets of class II and III obese patients.

METHODS

Fasting plasma levels of glucose, total cholesterol, LDL-c, HDL-c, triglycerides, free fatty acids, apoproteins A-I, B-100, B-48, C-II, C-III, insulin, hs-CRP, adipocytokines (adiponectin, resistin, leptin), unacylated ghrelin, body composition (DXA) and resting energy expenditure were measured in three subsets of obese patients: 17 metabolically abnormal obese (MAO) with metabolic syndrome and the typical metabolic dyslipidaemia, 21 metabolically healthy obese (MHO) without metabolic syndrome and with a normal lipid profile, and 21 isolated low HDL-c obese patients (LHO) without metabolic syndrome, compared to 21 healthy lean control subjects.

RESULTS

Insulin resistance (HOMA-IR) increased gradually from MHO to LHO and from LHO to MAO patients (p < 0.05 between MHO and MAO and between LHO and MAO). In multiple regression analysis, serum unacylated ghrelin levels were only positively and independently associated with HDL-c levels in the LHO group (p = 0.032).

CONCLUSIONS

These results suggest that, in class II and III obese patients with an isolated low HDL-c phenotype, unacylated ghrelin is positively associated with HDL-c level independently of insulin resistance and CRP levels, and may contribute to the highly prevalent low HDL-c level seen in obesity.

High-density lipoprotein metabolism in human apolipoprotein B(100) transgenic/brown adipose tissue deficient mice: a model of obesity-induced hyperinsulinemia

Obese and diabetic humans display decreased plasma high-density lipoprotein cholesterol (HDL-C) concentrations and an increased risk for coronary heart disease. However, investigation on HDL metabolism in obesity with a particular emphasis on hepatic ATP-binding cassette transporter A1 (ABCA1), the primary factor for HDL formation, has not been well studied. Human apolipoprotein B(100) transgenic (hApoB(tg)) and brown adipose tissue deficient (BATless) mice were crossed to generate hApoB(tg)/BATless mice. Male and female hApoB(tg) and hApoB(tg)/BATless mice were maintained on either a regular rodent chow diet or a diet high in fat and cholesterol until 24 weeks of age. The hApoB(tg)/BATless mice that were fed a HF/HC diet became obese, developed hepatic steatosis, and had significantly elevated plasma insulin levels compared with their hApoB(tg) counterparts, but plasma concentrations of total cholesterol, HDL-C, triglycerides, and free fatty acids and lipoprotein distribution between genotypes were not significantly different. Hepatic expression of genes encoding HDL-modifying factors (e.g., scavenger receptor, class B, type I, hepatic lipase, lecithin:cholesterol acyltransferase, and phospholipid transfer protein) was either altered significantly or showed a trend of difference between 2 genotypes of mice. Importantly, hepatic protein levels of ABCA1 were significantly lowered by ∼35% in male obese hApoB(tg)/BATless mice with no difference in mRNA levels compared with hApoB(tg) counterparts. Despite reduced hepatic ABCA1 protein levels, plasma HDL-C concentrations were not altered in male obese hApoB(tg)/BATless mice. The result suggests that hepatic ABCA1 may not be a primary contributing factor for perturbations in HDL metabolism in obesity-induced hyperinsulinemia.

Gene activation regresses atherosclerosis, promotes health, and enhances longevity

Background

Lifestyle factors and pharmacological compounds activate genetic mechanisms that influence the development of atherosclerotic and other diseases. This article reviews studies on natural and pharmacological gene activation that promotes health and enhances longevity.

Results

Living habits including healthy diet and regular physical activity, and pharmacotherapy, upregulate genes encoding enzymes and apolipoprotein and ATP-binding cassette transporters, acting in metabolic processes that promote health and increase survival. Cytochrome P450-enzymes, physiological factors in maintaining cholesterol homeostasis, generate oxysterols for the elimination of surplus cholesterol. Hepatic CTP:phosphocholine cytidylyltransferase-α is an important regulator of plasma HDL-C level. Gene-activators produce plasma lipoprotein profile, high HDL-C, HDL₂-C and HDL-C/cholesterol ratio, which is typical of low risk of atherosclerotic disease, and also of exceptional longevity together with reduced prevalence of cardiovascular, metabolic and other diseases. High HDL contributes to protection against inflammation, oxidation and thrombosis, and associates with good cognitive function in very old people. Avoiding unhealthy stress and managing it properly promotes health and increases life expectancy.

Conclusions

Healthy living habits and gene-activating xenobiotics upregulate mechanisms that produce lipoprotein pattern typical of very old people and enhance longevity. Lipoprotein metabolism and large HDL₂ associate with the process of living a very long life. Major future goals for health promotion are the improving of commitment to both wise lifestyle choices and drug therapy, and further the developing of new and more effective and well tolerated drugs and treatments.

Very low density lipoprotein metabolism and plasma adiponectin as predictors of high-density lipoprotein apolipoprotein A-I kinetics in obese and nonobese men

CONTEXT

Hypercatabolism of high-density lipoprotein (HDL) apolipoprotein (apo) A-I results in low plasma apoA-I concentration. The mechanisms regulating apoA-I catabolism may relate to alterations in very low density lipoprotein (VLDL) metabolism and plasma adiponectin and serum amyloid A protein (SAA) concentrations.

OBJECTIVE

We examined the associations between the fractional catabolic rate (FCR) of HDL-apoA-I and VLDL kinetics, plasma adiponectin, and SAA concentrations.

STUDY DESIGN

The kinetics of HDL-apoA-I and VLDL-apoB were measured in 50 obese and 37 nonobese men using stable isotopic techniques.

RESULTS

In the obese group, HDL-apoA-I FCR was positively correlated with insulin, homeostasis model of assessment for insulin resistance (HOMA-IR) score, triglycerides, VLDL-apoB, and VLDL-apoB production rate (PR). In the nonobese group, HDL-apoA-I FCR was positively correlated with triglycerides, apoC-III, VLDL-apoB, and VLDL-apoB PR and negatively correlated with plasma adiponectin. Plasma SAA was not associated with HDL-apoA-I FCR in either group. In multiple regression analyses, VLDL-apoB PR and HOMA-IR score, and VLDL-apoB PR and adiponectin were independently predictive of HDL-apoA-I FCR in the obese and nonobese groups, respectively. HDL-apoA-I FCR was positively and strongly associated with HDL-apoA-I PR in both groups.

CONCLUSIONS

Variation in VLDL-apoB production, and hence plasma triglyceride concentrations, exerts a major effect on the catabolism of HDL-apoA-I. Insulin resistance and adiponectin may also contribute to the variation in HDL-apoA-I catabolism in obese and nonobese subjects, respectively. We also hypothesize that apoA-I PR determines a steady-state, lowered plasma of apoA-I, which may reflect a compensatory response to a primary defect in the catabolism of HDL-apoA-I due to altered VLDL metabolism.

Obesity-related changes in high-density lipoprotein metabolism

Obesity is associated with a 3-or-more-fold increase in the risk of fatal and nonfatal myocardial infarction (1,2,3,4,5,6). The American Heart Association has reclassified obesity as a major, modifiable risk factor for coronary heart disease (7). The increased prevalence of premature coronary heart disease in obesity is attributed to multiple factors (8,9,10). A principal contributor to this serious morbidity is the alterations in plasma lipid and lipoprotein levels. The dyslipidemia of obesity is commonly manifested as high plasma triglyceride levels, low high-density lipoprotein cholesterol (HDLc), and normal low-density lipoprotein cholesterol (LDLc) with preponderance of small dense LDL particles (7,8,9,10). However, there is a considerable heterogeneity of plasma lipid profile in overweight and obese people. The precise cause of this heterogeneity is not entirely clear but has been partly attributed to the degree of visceral adiposity and insulin resistance. The emergence of glucose intolerance or a genetic predisposition to familial combined hyperlipidemia will further modify the plasma lipid phenotype in obese people (11,12,13,14,15).

Low serum high-density lipoprotein cholesterol in obese subjects with normal serum triglycerides: the role of insulin resistance and inflammatory cytokines

BACKGROUND

The cause of obesity-related low HDLc in the absence of hypertriglyceridemia is not known.

SUBJECTS & METHODS

A total of 32 subjects with a body mass index (BMI)(kg/m(2)) greater than 30 and normal serum triglycerides (<150 mg/dl) were identified.

RESULTS

People with low HDLc (n = 16) compared to those with normal HDLc (n = 16) had higher BMI (37.53 +/- 4.54 vs. 33.99 +/- 3.65 kg/m(2), p < 0.021), higher body fat weight (42.59 +/- 9.51 vs. 34.76 +/- 8.7 kg, p < 0.023), and higher insulin resistance index (3.75 +/- 2.51 vs. 1.95 +/- 1.10, p < 0.013). Seven subjects with low HDLc and none of those with normal HDLc, had elevated serum tumor necrosis factor alpha and/or interleukin-6 (p < 0.010).

CONCLUSIONS

Low HDLc levels can occur in obesity independently of elevated serum triglycerides and may be secondary to elevated serum levels of inflammatory cytokines.

Relationship between apolipoproteins and the alteration of HDL subclasses in hyperlipidemic subjects

BACKGROUND

To elucidate the relationship between the apolipoproteins, especially apoA-I and the alteration of HDL subclasses in hyperlipidemic, HTC and HTG subjects.

METHODS

ApoA-I contents of plasma HDL subclasses were quantitated by two-dimensional gel electrophoresis coupled with immunodetection in 233 normolipidemic subjects and 312 hyperlipidemic subjects (132 HTC and 180 HTG subjects). Making use of the mean +/-1 SD of apoA-I levels, we further subdivided normolipidemic, hyperlipidemic, HTC and HTG subjects into 3 subgroups, respectively.

RESULTS

Subjects in the middle and low apoA-I subgroups had decreased HDL-C and apoA-I while increased TG, apoB100, apoCII, apoCIII and apoE concentrations. With the reduction of apoA-I concentrations, the apoA-I contents of all HDL subclasses decreased successively and significantly. The relative percentage of small-sized HDL increased significantly while those of large-sized HDL(2a), HDL(2b) decreased significantly in hyperlipidemic, especially in HTG group. Multiple liner regression result revealed that apoA-I was positively and significantly correlated with all HDL subclasses and apoA-I level influenced the distribution of HDL subclasses powerfully in hyperlipidemic subjects.

CONCLUSIONS

Both the rate and efficiency of RCT might be weakened more seriously in hyperlipidemic, especially in HTG subjects with low apoA-I levels. ApoA-I level might be a powerful factor correlated with the distributions of HDL subclasses.

Fasting serum triglyceride and high-density lipoprotein cholesterol levels in patients intended to be treated for dyslipidemia

Objective

The aim of the present investigation was to evaluate the influence of serum triglycerides (TG) on other plasma lipids in patients to be treated for dyslipidemia.

Methodology

Lipid profiles of a cohort of 801 patients (487 males and 314 females) aged 57 ± 9 years (mean ± SD) were evaluated. Patients were stratified according to their plasma lipid levels. They were divided into various groups on the basis of serum TG (≥150 or <150 mg/dL) and high-density lipoprotein cholesterol (HDL-C) (≥40 or <40 mg/dL).

Results

Patients with TG ≥ 150 mg/dL had a higher total cholesterol and lower HDL-C levels compared with those with TG < 150 mg/dL, (p < 0.001). Patients with HDL-C < 40 mg/dL had a lower serum total cholesterol and higher TG compared with those with HDL-C ≥ 40 mg/dL (p = 0.011 and p < 0.0001, respectively). In all patients as well as in the subgroups, an inverse correlation between TG and HDL-C was found (r = −0.377, p < 0.001).

Conclusions

Although, the metabolic pathway for TG and HDL-C is closely linked, an inverse correlation between TG and HDL-C levels seems to exist in the entire sampled population. This correlation also appears to persist in fasting patients with low levels of TG.

High-density lipoprotein apolipoprotein A-I kinetics: comparison of radioactive and stable isotope studies

To compare the kinetic determinants of high-density lipoprotein (HDL) apolipoprotein A-I (apoA-I) concentration in lean normolipidaemic subjects using radioisotope and stable isotope studies. We pooled data from 16 radioisotope and 13 stable isotope studies to investigate the kinetics of apoA-I in lean normolipidemic individuals. We also examined the associations of HDL kinetic parameters with age, sex, body mass index (BMI) and concentrations of apoA-I, triglycerides, HDL cholesterol and low-density lipoprotein (LDL) cholesterol. Lean subjects from radioisotope and stable isotope studies were matched for age, gender, BMI and lipid profile. The apoA-I concentration was significantly lower in the radioisotope group than the stable isotope group (P = 0.031). There was no significant difference in HDL apoA-I fractional catabolic rate (FCR) and production rate (PR) between the groups. In the radioisotope group, HDL apoA-I FCR was significantly associated with apoA-I and HDL cholesterol concentrations (r = -0.681, P < 0.001 and r = -0.542, P < 0.001, respectively), whereas in the stable isotope group, only HDL apoA-I PR was significantly associated with apoA-I concentration (r = 0.455, P = 0.004). Our findings suggest that HDL apoA-I FCR is the primary determinant of apoA-I concentrations in lean subjects in studies using radiotracer techniques. By contrast, HDL apoA-I PR is the primary determinant of apoA-I concentration in lean subject in studies employing stable isotope methods. These discrepancies may be reconciled by differences in methodologies and/or study population characteristics.

Mechanisms of HDL lowering in insulin resistant, hypertriglyceridemic states: the combined effect of HDL triglyceride enrichment and elevated hepatic lipase activity

Hypertriglyceridemia, low plasma concentrations of high density lipoproteins (HDL) and qualitative changes in low density lipoproteins (LDL) comprise the typical dyslipidemia of insulin resistant states and type 2 diabetes. Although isolated low plasma HDL-cholesterol (HDL-c) and apolipoprotein A-I (apo A-I, the major apolipoprotein component of HDL) can occur in the absence of hypertriglyceridemia or any other features of insulin resistance, the majority of cases in which HDL-c is low are closely linked with other clinical features of insulin resistance and hypertriglyceridemia. We and others have postulated that triglyceride enrichment of HDL particles secondary to enhanced CETP-mediated exchange of triglycerides and cholesteryl ester between HDL and triglyceride-rich lipoproteins, combined with the lipolytic action of hepatic lipase (HL), are driving forces in the reduction of plasma HDL-c and apoA-I plasma concentrations. The present review focuses on these metabolic alterations in insulin resistant states and their important contributions to the reduction of HDL-c and HDL-apoA-I plasma concentrations.

Increased intra-abdominal fat may lower HDL levels by increasing the fractional catabolic rate of Lp A-I in postmenopausal women

High-density lipoprotein (HDL) particles without apolipoprotein A-II (Lp A-I) may be more anti-atherogenic than HDL with apo A-II (Lp A-I/AII) and Lp A-I is reported selectively to be reduced in cases of intra-abdominal obesity. We explored the mechanisms of this reduction by studying the turnover of Lp A-I and Lp A-I/A-II in postmenopausal women well characterized for total body, regional and sub-regional adiposity by body mass index (BMI), truncal girth ratio, and abdominal magnetic resonance imaging (MRI), respectively. We tested for possible cause-effect relationships by measuring inter-correlations among these variables. Intra-abdominal fat area correlated strongly and positively with the fractional catabolic rate (FCR) of Lp A-I (r=0.98, P=0.003). Intra-abdominal fat only showed a non-significant trend toward correlation with the FCR of Lp A-I/A-II (r=0.84, P=0.07), and had no correlation with the production or transport rate (TR) of either Lp A-I or Lp A-I/A-II (r=0.48 and 0.02, respectively, P>0.1). Subjects were studied both with and without estrogen replacement, allowing exploration of a possible interaction of adiposity with estrogen effects on HDL turnover. Response of HDL turnover to estrogen did not correlate with adiposity, except for a parameter of waist to hip ratio (WHR), which predicted the increase in LP A-I TR with estrogen (r=0.84, P=0.04). We conclude that intra-abdominal fat may lower HDL levels by increasing the FCR of Lp A-I, suggesting a mechanism by which central adiposity may be proatherogenic.

Apolipoprotein A-I and A-II kinetic parameters as assessed by endogenous labeling with [(2)H(3)]leucine in middle-aged and elderly men and women

The purpose of our study was to investigate high density lipoprotein (HDL) apolipoprotein (apo) A-I and apoA-II kinetics in a state of constant feeding after a primed-constant infusion of [5,5, 5-(2)H(3)]L-leucine in 32 normolipidemic older men and postmenopausal women (aged 41 to 79 years). ApoA-I and apoA-II were isolated from plasma HDL, and enrichment was determined by gas chromatography/mass spectrometry. The fractional secretion rate was obtained by using a monoexponential equation calculated with the SAAM II program (Department of Bioengineering, University of Washington, Seattle). Mean HDL cholesterol (HDL-C) and total triglyceride levels were 23% higher and 27% lower, respectively, in women than in men. Mean plasma apoA-I levels were 10% greater in women than in men, whereas mean apoA-II levels were similar. HDL size, estimated by gradient-sizing gels and by the HDL-C/apoA-I+apoA-II ratio, was significantly higher in women than in men. Mean apoA-I secretion rates (SRs) were similar in men and women (12.28+/-3.64 versus 11.96+/-2.92 mg/kg per day), whereas there was a trend toward a lower (-13%) apoA-I fractional catabolic rate (FCR) in women compared with men (0.199+/-0.037 versus 0. 225+/-0.062 pools per day, P=0.11). Mean apoA-II SRs (2.21+/-0.57 versus 2.27+/-0.91 mg/kg per day) and FCRs (0.179+/-0.034 versus 0. 181+/-0.068 pools per day) were similar in men and women. For the group as a whole, there was an inverse association between the HDL-C/apoA-I+apoA-II ratio and apoA-I FCR and between the ratio and triglyceride levels. Plasma levels of apoA-I and apoA-II were correlated with their respective SRs but not FCRs. These data suggest a major role for apoA-I and apoA-II SRs in regulating the plasma levels of these proteins, whereas apoA-I FCR might be an important factor contributing to the differences in apoA-I levels between men and postmenopausal women. Moreover, plasma triglyceride levels are important determinants of HDL size and apoA-I catabolism.

Cubilin, the endocytic receptor for intrinsic factor-vitamin B(12) complex, mediates high-density lipoprotein holoparticle endocytosis

Receptors that endocytose high-density lipoproteins (HDL) have been elusive. Here yolk-sac endoderm-like cells were used to identify an endocytic receptor for HDL. The receptor was isolated by HDL affinity chromatography and identified as cubilin, the recently described endocytic receptor for intrinsic factor-vitamin B(12). Cubilin antibodies inhibit HDL endocytosis by the endoderm-like cells and in mouse embryo yolk-sac endoderm, a prominent site of cubilin expression. Cubilin-mediated HDL endocytosis is inhibitable by HDL(2), HDL(3), apolipoprotein (apo)A-I, apoA-II, apoE, and RAP, but not by low-density lipoprotein (LDL), oxidized LDL, VLDL, apoC-I, apoC-III, or heparin. These findings, coupled with the fact that cubilin is expressed in kidney proximal tubules, suggest a role for this receptor in embryonic acquisition of maternal HDL and renal catabolism of filterable forms of HDL.

HDL metabolism in hypertriglyceridemic states: an overview

Reduced plasma high-density lipoprotein (HDL) cholesterol levels have been recognized as a highly significant independent risk factor for atherosclerotic cardiovascular disease. HDL levels are also inversely related to plasma triglyceride levels and there is a dynamic interaction between HDL and triglyceride (TG) rich lipoproteins in vivo. The mechanisms underlying the lowering of HDL in hypertriglyceridemic states have not been fully elucidated, but there is evidence to suggest that triglyceride enrichment of HDL, a common metabolic consequence of hypertriglyceridemia, may play an important role in this process. There is accumulating evidence to suggest that the primary mechanisms leading to reduced plasma HDL cholesterol levels and HDL particle number in hypertriglyceridemic states may be due to any one or a combination of the following possibilities: (1) small HDL particles, which are the product of the intravascular lipolysis of triglyceride-enriched HDL, may be cleared more rapidly from the circulation, (2) triglyceride-enriched HDL may be intrinsically more unstable in the circulation, with apo A-I loosely bound, (3) the lipolytic process itself of triglyceride-enriched HDL may lower HDL particle number by causing apo A-I to be shed from the HDL particles and cleared from the circulation, (4) a dysfunctional lipoprotein lipase or reduced LPL activity may contribute to the lowering of HDL levels by reducing the availability of surface constituents of triglyceride-rich lipoproteins that are necessary for the formation of nascent HDL particles. This review summarizes the evidence that triglyceride-enrichment of HDL is an important factor determining the rate at which HDL is catabolized, a mechanism which could explain, at least in part, the reduced plasma HDL cholesterol levels and particle number frequently observed in hypertriglyceridemic states.

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.

Increased high density lipoprotein (HDL), defective hepatic catabolism of ApoA-I and ApoA-II, and decreased ApoA-I mRNA in ob/ob mice. Possible role of leptin in stimulation of HDL turnover

Abnormalities of plasma high density lipoprotein (HDL) levels commonly reflect altered metabolism of the major HDL apolipoproteins, apoA-I and apoA-II, but the regulation of apolipoprotein metabolism is poorly understood. Two mouse models of obesity, ob/ob and db/db, have markedly increased plasma HDL cholesterol levels. The purpose of this study was to evaluate mechanisms responsible for increased HDL in ob/ob mice and to assess potential reversibility by leptin administration. ob/ob mice were found to have increased HDL cholesterol (2-fold), apoA-I (1.3-fold), and apoA-II (4-fold). ApoA-I mRNA was markedly decreased (to 25% of wild-type) and apoA-II mRNA was unchanged, suggesting a defect in HDL catabolism. HDL apoprotein turnover studies using nondegradable radiolabels confirmed a decrease in catabolism of apoA-I and apoA-II and a 4-fold decrease in hepatic uptake in ob/ob mice compared with wild-type, but similar renal uptake. Low dose leptin treatment markedly lowered HDL cholesterol and apoA-II levels in both ob/ob mice and in lean wild-type mice, and it restored apoA-I mRNA to normal levels in ob/ob mice. These changes occurred without significant alteration in body weight. Moreover, ob/ob neuropeptide Y-/- mice, despite marked attenuation of diabetes and obesity phenotypes, showed no change in HDL cholesterol levels relative to ob/ob mice. Thus, increased HDL levels in ob/ob mice reflect a marked hepatic catabolic defect for apoA-I and apoA-II. In the case of apoA-I, this is offset by decreased apoA-I mRNA, resulting in apoA-II-rich HDL particles. The studies reveal a specific HDL particle catabolic pathway that is down-regulated in ob/ob mice and suggest that HDL apolipoprotein turnover may be regulated by obesity and/or leptin signaling.

The atherogenic significance of an elevated plasma triglyceride level

The importance of hypertriglyceridemia as an independent predictor of coronary artery disease (CAD) remains unsettled. Hypertriglyceridemia, with or without associated hypercholesterolemia, occurs more frequently in premature CAD subjects than does hypercholesterolemia alone. With univariate analysis, most studies show a positive correlation between plasma triglyceride (TG) level and risk for CAD, but with multivariate analysis plasma TG level is no longer an independent risk factor except in women and diabetics. Prospective studies have shown that subjects with a high LDL/HDL cholesterol ratio and a high plasma TG level have the highest risk for CAD. Hypertriglyceridemia signifies the presence of excess triglyceride-rich lipoproteins (TRL), including chylomicrons, VLDL, and their remnants. The question then becomes one of whether TRL are directly or indirectly involved in atherogenesis. TRL were thought to be too big to infiltrate the arterial wall, and histopathological studies have shown cholesterol but not triglyceride accumulation in the atherosclerotic plaque. However, there was a recent demonstration of undegraded VLDL and IDL in atherosclerotic plaques. Larger TRL may undergo hydrolysis on the arterial surface to become smaller particles before entry into the intima. Possible cellular pathways for the uptake of TRL by macrophages have been described. The smaller TRL (Sf 20-60), including postprandial chylomicron remnants, are believed to be the most atherogenic of all TRL particles. Because large amounts of TRL are produced in the postprandial period, atherogenesis involving TRL may be primarily a postprandial phenomenon. Once in the intima, TG may undergo hydrolysis, releasing free fatty acids and mono- and diacyl glycerol, accounting for the dearth of TG in atherosclerotic lesions. Particle for particle, VLDL delivers five times as much cholesterol as LDL does to the macrophage.

VLDL compositional changes and plasma levels of triglycerides and high density lipoprotein

VLDL chemical composition is related to plasma levels of triglycerides and HDL-cholesterol. We evaluated patients with primary hypertriglyceridemia with or without hypoalphalipoproteinemia and subjects with normotriglyceridemia with hypoalphalipoproteinemia. The pattern observed in all the groups was an enrichment in the triglyceride content of VLDL and in apo B-VLDL. Compared to controls, LpC-III:B levels were higher in hypertriglyceridemic patients with low or normal HDL-cholesterol levels (7.3 +/- 0.6 vs. 14.9 +/- 1.8 and 12.3 +/- 2.8 mg/dl; P < 0.005 and P < 0.01, respectively) and LpE:B concentration was only increased in patients with hypertriglyceridemia and normal HDL-cholesterol levels (3.1 +/- 0.5 vs. 6.3 +/- 1.0 mg/dl; P < 0.01). The activity of the cholesteryl ester transfer protein was higher in hypertriglyceridemic patients with low HDL-cholesterol levels than in controls (380 +/- 25 vs. 262 +/- 14% cholesteryl esters/ml.h; P < 0.001). The most atypical VLDL particle was found in patients who combined an accumulation of VLDL particles and a reduction in HDL-cholesterol concentration. These two parameters represent both ends of the cholesteryl ester-triglyceride transfer, a crucial factor for VLDL chemical composition and HDL levels.

In vivo kinetics as a sensitive method for testing physiologically intact human recombinant apolipoprotein A-I: comparison of three different expression systems

In order to assess the structural and functional integrity of recombinant human apoA-I, we expressed apoA-I using three different expression systems: Baculovirus transfected Spodoptera frugiperda (Sf9) cells, stably transfected Chinese hamster ovary (CHO) cells, and transformed Escherichia coli (E. coli). Purified apoA-I from the three expression systems was radioiodinated and their catabolism was compared in normolipemic rabbits. The kinetic turnover studies of radiolabelled apoA-I in normolipemic rabbits revealed that highly purified recombinant apoA-I had an identical decay curve compared to native apoA-I, regardless whether it was purified from Sf9 cells, CHO cells, or E. coli. We also determined the association of the three recombinant apoA-I forms with both rabbit and human HDL. All three recombinant apoA-I forms were associated with HDL2 and HDL3 after injection into the rabbits and after incubation with human serum using both a Superose 6 column separation system and density gradient ultracentrifugation. The addition of the pro-segment or the addition of methionine at the amino-terminal end of apoA-I did not alter its metabolism and association to HDL. In conclusion, all studied expression systems are capable of producing high levels of physiologically intact recombinant human apoA-I. The aminoterminal addition of the prosegment of apoA-I or methionine did not alter the in vivo metabolism of apoA-I or its association to HDL.

Role of the carboxy-terminal domain of human apolipoprotein AI in high-density-lipoprotein metabolism--a study based on deletion and substitution variants in transgenic mice

Cholesterol levels in high-density lipoprotein (HDL) of transgenic mice overexpressing human apolipoprotein AI (apoAI), a des-(190-243)-apoAI deletion mutant or an apoAI-(1-189)-apoAII-(12-77) chimera were 2.8-fold (P<0.001), 1.3-fold (P<0.05) and 2.2-fold (P<0.001) higher than in control mice, respectively. Human apolipoprotein levels in apoAI and in apoAI-(1-189)-apoAII-(12-77) transgenic mice were 5.2-fold and 3.5-fold higher than in des-(190-243)-apoAI transgenic mice, whereas their HDL cholesterol levels were 2.1-fold and 1.6-fold higher. PAGE of HDL isolated by ultracentrifugation revealed that murine HDL migrated as 9.6-nm and 7.2-nm particles. Overexpression of human apoAI and apoAI-(1-189)-apoAII-(12-77) resulted in the production of polydisperse HDL (9.6, 9.2, 8.4 and 7.2 nm) particles, whereas overexpression of des-(190-243)-apoAI primarily resulted in an increase of 7.2-nm particles. The fractional catabolic rates of human apoAI and apoAI-(1-189)-apoAII-(12-77) were very similar, whereas that of des-(190-243)-apoAI was 4.9-fold higher. The endogenous production rates of human apoAI, des-(190-243)-apoAI and apoAI-(1-189)-apoAII-(12-77) in transgenic mice were very similar. It is concluded that deletion of the carboxy-terminal domain of apoAI reduces its lipoprotein association, resulting in the production of small, phospholipid-rich HDL particles that are cleared more rapidly. Substitution of the carboxy-terminal helices of apoAI with helices of apoAII restores lipoprotein association, resulting in the production of HDL, which migrates as human HDL3 and HDL2. Although the carboxy-terminal domain of the chimera contained more than 80% of the amino acid sequence of apoAII, its HDL-distribution profile in transgenic mice was very similar to that of human apoAI. This study demonstrates the importance of the helical structure of apoAI of the carboxy-terminal domain of apoAI, rather than of its exclusive amino acid sequence, in HDL metabolism.

Effects of deletion of the carboxyl-terminal domain of ApoA-I or of its substitution with helices of ApoA-II on in vitro and in vivo lipoprotein association

In the present study, the lipoprotein association of apoA-I, an apoA-I (DeltaAla190-Gln243) deletion mutant and an apoA-I (Asp1-Leu189)/apoA-II (Ser12-Gln77) chimera were compared. At equilibrium, 80% of the 125I-labeled apolipoproteins associated with lipoproteins in rabbit or human plasma but with very different distribution profiles. High density lipoprotein (HDL)2,3-associated fractions were 0.60 for apoA-I, 0.30 for the chimera, and 0.15 for the deletion mutant, and corresponding very high density lipoprotein-associated fractions were 0.20, 0.50, and 0.65. Clearance curves after intravenous bolus injection of 125I-labeled apolipoproteins (3 microg/kg) in normolipemic rabbits could be adequately fitted with a sum of three exponential terms, yielding overall plasma clearance rates of 0.028 +/- 0.0012 ml.min-1 for apoA-I (mean +/- S.E.; n = 6), 0.10 +/- 0.008 ml.min-1 for the chimera (p < 0.001 versus apoA-I) and 0.38 +/- 0.022 ml.min-1 for the deletion mutant (p < 0.001 versus apoA-I and versus the chimera). Fractions that were initially cleared with a t1/2 of 3 min, most probably representing free apolipoproteins, were 0.30 +/- 0.04, 0.50 +/- 0.06 (p = 0.02 versus apoA-I), and 0.64 +/- 0.07 (p = 0.002 versus apoA-I), respectively. At 20 min after the bolus, the fractions of injected material associated with HDL2,3 were 0.55 +/- 0.06, 0.25 +/- 0.03 (p = 0.001 versus apoA-I), and 0.09 +/- 0.01 (p < 0.001 versus apoA-I and versus the chimera), respectively, whereas the fractions associated with very high density lipoprotein were 0. 15 +/- 0.006, 0.25 +/- 0.03 (p = 0.008 versus apoA-I), and 0.27 +/- 0.03 (p = 0.003 versus apoA-I), respectively. The ability of the different apolipoproteins to bind to HDL3 particles and displace apoA-I in vitro were compared. The molar ratios at which 50% of 125I-labeled apoA-I was displaced from the surface of HDL3 particles were 1:1 for apoA-I, 3:1 for the chimera and 12:1 for the deletion mutant, indicating 3- and 12-fold reductions of the affinities for HDL3 of the chimera and the deletion mutant, respectively. These data suggest that the carboxyl-terminal pair of helices of apoA-I are involved in the initial rapid binding of apoA-I to the lipid surface of HDL. Although the lipid affinity of apoA-II is higher than that of apoA-I, substitution of the carboxyl-terminal helices of apoA-I with those of apoA-II only partially restores its lipoprotein association. Thus, this substitution may affect cooperative interactions with the middle amphipathic helices of apoA-I that are critical for its specific distribution over the different HDL species.

Changes in blood lipid composition and response to interferon treatment in chronic hepatitis C

To assess whether the initial status of lipid metabolism in patients with chronic viral hepatitis might correlate with outcome of therapy, 52 patients (32 males and 20 female) with chronic hepatitis C were studied: 44 were treated with human recombinant interferon-alpha 2b (3 MU three times per week for up to 12 months), and 8 served as controls. At baseline, sera were tested for total and HDL cholesterol, HDL2, HDL3, apolipoprotein A-I, apolipoprotein B, interferon-alpha, tumor necrosis factor, and interleukin-6. Changes in blood lipids were evaluated after 3, 30, and 90 days of treatment. HDL cholesterol, apolipoprotein A-I, and HDL3 decreased by 9.4-11.4% within 4 weeks of starting interferon treatment, but this effect was sustained only in patients with a primary response to interferon. On multivariate analysis, a primary response to interferon correlated with higher apolipoprotein A-I and lower (< 2.23 pg/ml) interleukin-6 levels (p < 0.005 for both). In contrast, a sustained response was significantly more common in patients with low (< or = 13.3 pg/ml) serum interferon-alpha and lower interleukin-6 at baseline but did not correlate with any of the blood lipids. Thus, in chronic hepatitis C, interferon treatment induces specific changes in blood lipids. The concentration of apolipoprotein A-I at baseline is a strong predictor of primary response to treatment, and the likelihood of sustained response seems to be reflected by lower cytokine activation.

Severe familial HDL deficiency in French-Canadian kindreds. Clinical, biochemical, and molecular characterization

A decreased level of HDL cholesterol (HDL-C) is the most common lipoprotein abnormality seen in people with premature coronary artery disease (CAD). In many cases, HDL-C reduction in patients with CAD may be the result of increased apo B-containing lipoprotein production by the liver with secondary hypoalphalipoproteinemia. Primary hypoalphalipoproteinemia is seen in approximately 4% of people with CAD. We report findings in four subjects with severe familial HDL deficiency (HDL-C << 5th percentile for age and sex; 0.08 to 0.38 mmol/L) in three French-Canadian kindreds with autosomal codominant inheritance. By inclusion criteria, all four subjects had normal fasting triglycerides and none were diabetic. HDL particle size by gradient gel electrophoresis revealed small HDL particles (estimated Stokes' diameter, 8.14 to 8.30 nm). Apo AI analysis by polyacrylamide gel electrophoresis and use of isoelectrofocusing gels in affected subjects revealed normal molecular weight (28.3 kD) and normal isoelectrofocusing point but a relative increase in proapoliprotein AI, with near-normal levels of proapolipoprotein AI in plasma, suggesting normal secretion of apo AI. Quantitative Southern blot analysis of the apo AI-CIII-AIV gene cluster reveals no gene rearrangements or allele deletion. Haplotypes of the apo AI gene, determined by use of the restriction enzymes Pst I, Xmn I, and Sst I and of the apo AII gene by use of the enzyme Msp I, did not reveal segregation of the low HDL-C trait with either the apo AI or the AII gene. Sequence analysis of the promoter region of the apo AI gene reveals heterozygosity for guanine-to-adenine substitution at position 76 in two kindreds with no evidence of segregation with the low HDL trait. None of the patients had mutations of the lipoprotein lipase gene common in subjects of French-Canadian descent. Haplotype analysis of the lipoprotein lipase gene did not show segregation with the low HDL trait. Plasma lecithin: cholesterol acyltransferase (LCAT) activity was found to be within normal levels in affected subjects and in nonaffected first-degree relatives. None of the affected subjects had clinical manifestations of Tangier disease. Two of the four cases examined, both men, had severe CAD and had undergone revascularization procedures. The third is a younger brother of one of these probands and the fourth is a 30-year-old woman, and both were free of clinical CAD. However, in none of the families did the low HDL trait unequivocally cosegregate with CAD.(ABSTRACT TRUNCATED AT 400 WORDS)

Carboxyl-terminal domain truncation alters apolipoprotein A-I in vivo catabolism

Apolipoprotein A-I (apoA-I), the major protein of high density lipoproteins, facilitates reverse cholesterol transport from peripheral tissue to liver. To determine the structural motifs important for modulating the in vivo catabolism of human apoA-I (h-apoA-I), we generated carboxyl-terminal truncation mutants at residues 201 (apoA-I201), 217 (apoA-I217), and 226 (apoA-I226) by site-directed mutagenesis. ApoA-I was expressed in Escherichia coli as a fusion protein with the maltose binding protein, which was removed by factor Xa cleavage. The in vivo kinetic analysis of the radioiodinated apoA-I in normolipemic rabbits revealed a markedly increased rate of catabolism for the truncated forms of apoA-I. The fractional catabolic rates (FCR) of 9.10 +/- 1.28/day (+/- S.D.) for apoA-I201, 6.34 +/- 0.81/day for apoA-I217, and 4.42 +/- 0.51/day for apoA-I226 were much faster than the FCR of recombinant intact apoA-I (r-apoA-I, 0.93 +/- 0.07/day) and h-apoA-I (0.91 +/- 0.34/day). All the truncated forms of apoA-I were associated with very high density lipoproteins, whereas the intact recombinant apoA-I (r-apoA-I) and h-apoA-I associated with HDL2 and HDL3. Gel filtration chromatography revealed that in contrast to r-apoA-I, the mutant apoA-I201 associated with a phospholipid-rich rabbit apoA-I containing particle. Analysis by agarose gel electrophoresis demonstrated that the same mutant migrated in the pre-beta position, but not within the alpha position as did r-apoA-I. These results indicate that the carboxyl-terminal region (residue 227-243) of apoA-I is critical in modulating the association of apoA-I with lipoproteins and in vivo metabolism of apoA-I.

Familial lipoprotein disorders and premature coronary artery disease

Significant risk factors for premature coronary heart disease include: (1) family history, (2) elevated low density lipoprotein (LDL) cholesterol level > or = 160 mg/dl, l, (3) decreased high density lipoprotein (HDL) cholesterol level < 35 mg/dl, l, (4) cigarette smoking, (5) high blood pressure and (6) diabetes mellitus. All of these risk factors are common in patients with premature heart disease. Common familial lipid disorders associated with premature heart disease include familial lipoprotein(a) excess, familial dyslipidemia (elevated triglycerides and decreased HDL cholesterol), familial combined hyperlipidemia (elevations of LDL cholesterol and triglycerides, and often decreased HDL cholesterol), familial hypoapobetalipoproteinemia (elevated apolipoprotein B levels), familial hypoalphalipoproteinemia (low HDL cholesterol levels), and familial hypercholesterolemia (elevated LDL cholesterol levels). All these disorders have been characterized using age and gender specific 90th and 10th percentile values from the normal population. The diagnosis and potential management of these disorders is reviewed.

Human HDL cholesterol levels are determined by apoA-I fractional catabolic rate, which correlates inversely with estimates of HDL particle size. Effects of gender, hepatic and lipoprotein lipases, triglyceride and insulin levels, and body fat distribution

High-density lipoprotein (HDL) cholesterol (HDL-C) levels are a strong inverse predictor of atherosclerosis risk, but the physiological determinants of HDL-C levels are poorly understood. We selected 57 human subjects (30 women and 27 men) with a broad range of HDL-C levels and performed turnover studies of apolipoprotein (apo)A-I and apoA-II, the two major apolipoproteins of HDL, to measure the fractional catabolic rate (FCR) and production or transport rate (TR) of these proteins. We also measured several other parameters known to correlate with HDL-C levels to test for their interrelations and to postulate mechanisms of regulation of HDL-C levels. As expected, the women had higher levels of HDL-C (56.7 +/- 21.4 versus 45.1 +/- 16.3 mg/dL, mean +/- SD; P = .03) and apoA-I (147 +/- 32 versus 126 +/- 29 mg/dL, P = .01) than men and did not differ in apoA-II levels (34.5 +/- 7.4 versus 33.3 +/- 7.5 mg/dL, P > .2). The FCR of apoA-I tended to be lower in the women (0.248 +/- 0.077 versus 0.277 +/- 0.069 pools/d, P = .1), although the difference was not statistically significant. The FCR of apoA-II was also lower (0.184 +/- 0.043 versus 0.216 +/- 0.056 pools/d, P = .02). In contrast, the apoA-I TR was equal in women and men (12.0 +/- 1.6 versus 12.1 +/- 2.8 mg/kg per day, P > .2), and there was a trend toward lower apoA-II TR in women (2.19 +/- .62 versus 2.61 +/- 1.06 mg/kg per day, P = .07). Linear regression analysis revealed a strong inverse correlation between HDL-C levels and the FCRs of apoA-I and apoA-II (r = -.81 and -.76, respectively; P < .0001 for both). In contrast, there was little or no association between HDL-C and the TRs of apoA-I and apoA-II (r = .06 and -.35, P = not significant and .01, respectively). In stepwise multiple linear regression analysis, apoA-I FCR alone accounted for 66% of the variability in HDL-C; two other variables accounted for an additional 7%. Due to the importance of apoA-I FCR, its determinants were sought among the remaining variables.(ABSTRACT TRUNCATED AT 400 WORDS)

Homozygous Tangier disease and cardiovascular disease

Decreased levels of plasma high density lipoprotein (HDL) cholesterol have been associated with premature cardiovascular disease (CVD). Tangier disease is an autosomal co-dominant disorder in which homozygotes have a marked deficiency of HDL cholesterol and apolipoprotein (apo) A-I levels (both < 10 mg/dl), decreased low density lipoprotein (LDL) cholesterol levels (about 40% of normal), and mild hypertriglyceridemia. Homozygotes develop cholesterol ester deposition in tonsils (orange tonsils), liver, spleen, gastrointestinal tract, lymph nodes, bone marrow, and Schwann cells. Our purpose was to assess the prevalence of CVD in Tangier disease. We reviewed published clinical information on 51 cases of homozygous Tangier disease, report 3 new cases and provide autopsy information on 3 cases. Mean (+/- S.D.) lipid values of all cases were as follows: total cholesterol 68 +/- 30 mg/dl (32% of normal), triglycerides 201 +/- 118 mg/dl (162% of normal), HDL cholesterol 3 +/- 3 mg/dl (6% of normal) and LDL cholesterol 50 +/- 38 mg/dl (37% of normal). The most common clinical finding in these subjects (n = 54) was peripheral neuropathy which was observed in 54% of cases versus < 1% of control subjects (n = 3130). CVD was observed in 20% of Tangier patients versus 5% of controls (P < 0.05), and in those that were between 35 and 65 years of age, 44% (11 of 25) had evidence of CVD (either angina, myocardial infarction or stroke) versus 6.5% in 1533 male controls and 3.2% in 1597 female controls in this age group (P < 0.01). In 9 patients who died, 2 died prior to age 20 of probable infectious diseases, 3 of documented coronary heart disease at ages 48, 64, and 72, 2 of stroke at ages 56 and 69, one of valvular heart disease, and 1 of cancer. In three autopsy cases, significant diffuse atherosclerosis was observed in one at age 64, moderate atherosclerosis and cerebral infarction in another at age 56, but no atherosclerosis was noted in the third case who died of lymphoma at age 62. In one patient with established coronary heart disease, none of the lipid lowering agents used (niacin, gemfibrozil, estrogen or lovastatin) raised HDL cholesterol levels above 5 mg/dl. However, these agents did have significant effects on lowering triglyceride and LDL cholesterol levels. Our data indicate that there may be heterogeneity in these patients with regard to CVD risk, that peripheral neuropathy is a major problem in many patients, and that CVD is a significant clinical problem in middle aged and elderly Tangier homozygotes.(ABSTRACT TRUNCATED AT 400 WORDS)

Lipoprotein metabolism and its relationship to atherosclerosis

Lipoproteins are macromolecular complexes that transport cholesterol and triglycerides through the blood stream. The assembly and secretion of lipoproteins occurs in the liver and small intestine, but many modifications and transformations occur in the plasma. Plasma levels of cholesterol and triglycerides are regulated by both environmental effects on lipid metabolism and by genetic factors affecting both apoproteins on the surface of the lipoproteins and enzymes in plasma. Abnormalities of the lipoprotein transport system can increase an individual's risk for developing atherosclerosis.

Familial lipoprotein disorders and premature coronary artery disease

Although there is consensus that lipid variables, especially lipoprotein(a), are heritable and that elevated LDL cholesterol levels should be treated, there are no clear definitions of the common familial lipid disorders associated with premature CHD (lipoprotein(a) excess, FCH, familial dyslipidemia, familial hypoalphalipoproteinemia, familial hypercholesterolemia), nor do we have clear guidelines for the treatment of most of these disorders. Implementation of therapy for elevated LDL cholesterol in familial lipid disorders often has not occurred even in the United States. Before recommendations can be made for subjects with lipoprotein(a) excess and HDL deficiency (who often have combined hyperlipidemia or hypertriglyceridemia), prospective studies documenting benefit of CHD risk reduction must be carried out in subjects with lipoprotein(a) excess and HDL deficiency. One such study is being carried out with gemfibrozil in CHD patients with HDL deficiency. Current data do justify treatment of CHD patients with lipoprotein(a) excess with niacin because niacin has been shown to lower lipoprotein(a) levels as well as lower CHD risk mortality in random CHD patients. With regard to CHD patients with or without HDL cholesterol levels less than 35 mg/dL (0.9 mmol/L), efforts should be made to optimize their lipid profile and reduce their LDL cholesterol levels to less than 100 mg/dL (2.6 mmol/L).

Drug control of reverse cholesterol transport

Reverse cholesterol transport identifies a series of metabolic events resulting in the transport of excess cholesterol from peripheral tissues to the liver. High-density lipoproteins (HDL) are the vehicle of cholesterol in this reverse transport, a function believed to explain the inverse correlation between plasma HDL levels and atherosclerosis. An attempt to stimulate, by the use of drugs, this transport process may hold promise in the prevention and treatment of arterial disease. Among the agents affecting lipoprotein metabolism, only probucol exerts significant effects on reverse cholesterol transport, by stimulating the activity of the cholesteryl ester transfer protein and, consequently, altering HDL subfraction composition/distribution. Another approach to the stimulation of reverse cholesterol transport consists of raising plasma HDL levels; studies in animals, either by exogenous supplementation or by endogenous overexpression, have shown a consistent benefit in terms of atherosclerosis regression and/or non-progression. Thus, it is time to consider different future treatments of atherosclerosis, combining the classical lipid-lowering treatments with innovative methods to promote cholesterol removal from the arterial wall.

Increased production rates of LDL are common in individuals with low plasma levels of HDL cholesterol, independent of plasma triglyceride concentrations

Reduced plasma levels of high density lipoprotein (HDL) cholesterol are associated with increased risk for coronary heart disease. Although plasma HDL levels are, in general, inversely related to plasma triglyceride (TG) concentrations, a small proportion of individuals with low HDL cholesterol concentrations have normal plasma TG levels. We wished to determine whether subjects with low plasma levels of HDL cholesterol could be characterized by common abnormalities of lipoprotein metabolism independent of plasma TGs. Therefore, we studied the metabolism of low density lipoprotein (LDL) apolipoprotein B (apo B) and HDL apolipoprotein A-I (apo A-I) in subjects with low plasma HDL cholesterol concentrations with or without hypertriglyceridemia. Nine subjects with low plasma HDL cholesterol levels and normal levels of plasma TGs and LDL cholesterol were studied. Autologous 131I-LDL and 125I-HDL were injected intravenously, and blood samples were collected for 2 weeks. LDL apo B and HDL apo A-I levels were measured by specific radioimmunoassays. Fractional catabolic rates (FCRs, pools per day) and production rates (PRs, milligrams/kilogram.day) for each apolipoprotein were determined. The results were compared with those obtained previously in nine subjects with low plasma HDL cholesterol levels and hypertriglyceridemia and in seven normal subjects. The normal subjects had an HDL apo A-I FCR (mean +/- SD) of 0.21 +/- 0.04. Despite large differences in plasma TG levels, the HDL apo A-I FCRs were similar in the low-HDL, normal-TG group (0.30 +/- 0.09) and the low-HDL, high-TG group (0.33 +/- 0.10), although only the latter value was significantly increased versus control subjects (p < 0.03). Increased apo A-I FCRs were associated with reduced HDL apo A-I levels in both groups of patients. Apo A-I PRs were similar in all groups. In contrast, LDL apo B PR was increased approximately 50% in the low-HDL, normal-TG group (19.3 +/- 6.6; p < 0.01) compared with normal subjects (12.5 +/- 2.6). There was a strong trend toward a greater LDL apo B PR in the low-HDL, high-TG group (17.6 +/- 4.5; p = 0.06 versus normal subjects) as well. LDL apo B FCRs were similar in all three groups. LDL apo B concentrations were also increased in the group with low HDL cholesterol and normal TG levels. Both groups with low HDL cholesterol levels had cholesterol-depleted LDL and HDL particles.(ABSTRACT TRUNCATED AT 400 WORDS)

Increased plasma and renal clearance of an exchangeable pool of apolipoprotein A-I in subjects with low levels of high density lipoprotein cholesterol

Plasma levels of HDL apo A-I are reduced in individuals with low HDL cholesterol (HDL-C) concentrations as a result of increased fractional catabolic rates (FCRs). To determine the basis for the high apo A-I FCRs, seven subjects with low HDL-C levels (31.0 +/- 4.3 mg/dl) were compared with three subjects with high HDL-C levels (72.0 +/- 4.5 mg/dl). Each subject received autologous HDL that was labeled directly by the iodine-monochloride method (whole-labeled) and autologous HDL that was labeled by exchange with homologous radiolabeled apo A-I (exchange-labeled). Blood was obtained for 2 wk, specific activities determined, and FCRs (d-1 +/- SD) estimated. In every subject, whether in the low or high HDL-C group, the exchange-labeled FCR was greater than the whole-labeled FCR. The exchange-labeled FCR was higher in the low HDL-C group (0.339 +/- 0.043) versus the high HDL-C group (0.234 +/- 0.047; P < 0.009). The whole-labeled FCR was also greater in the low HDL-C group (0.239 +/- 0.023) versus the high HDL-C group (0.161 +/- 0.064; P < 0.02). In addition, in both low and high HDL groups ultracentrifugation resulted in more radioactivity in d > 1.210 (as percentage of total plasma counts per minute) with the exchange-labeled tracer than with the whole-labeled tracer (12.55 +/- 4.95% vs. 1.02 +/- 0.38%; P < 0.003). With both HDL tracers, more radioactivity was found in d > 1.210 in the low versus the high HDL-C groups. When apo A-I catabolism was studied by perfusing isolated rabbit kidneys with whole-labeled HDL, there was twice as much accumulation (cpm/g cortex) of HDL apo A-I isolated from subjects with low HDL-C than from subjects with high HDL-C (P < 0.0025). Finally, HDL that had been isolated from subjects with high levels of HDL-C was triglyceride enriched and exposed to partially purified lipases before perfusion through kidneys. Threefold more apo A-I from modified HDL accumulated in the cortex compared with the unmodified preparation (P < 0.007). The results of these in vivo and ex vivo studies indicate that individuals with low HDL-C levels have more loosely bound, easily exchanged apo A-I and that this exchangeable apo A-I is more readily cleared by the kidney.

Decreased postprandial response to a fat meal in normotriglyceridemic men with hypoalphalipoproteinemia

The plasma level of high density lipoprotein cholesterol (HDL-C) has been reported to be inversely correlated with the level of triglycerides (TGs) and the magnitude of postprandial lipemia because subjects with low HDL-C accompanying high TG levels often have an increased postprandial response to a fat load. However, information is limited regarding the postprandial response to a fat load in subjects with low HDL-C and normal fasting TG values (hypoalphalipoproteinemia [hypoalpha]). We administered an oral fat load (70 g/m2 of body surface area) to six subjects with hypoalpha and six aged-matched control subjects. Plasma levels of lipids and lipoproteins and the mass of triglyceride-rich lipoproteins (TRLs: Sf > 100 and Sf 20-100) were measured every 2 hours for 8 hours. The mass and chemical composition of Sf > 100 or Sf 20-100 TRLs were not different between the fasting groups; postprandial Sf > 100 but not Sf 20-100 TRLs (mean +/- SD) was significantly lower in subjects with hypoalpha (200.4 +/- 64.8 mg/dL versus 110.6 +/- 50.9 mg/dL; analysis of variance, F test, p = 0.04). In the hypoalpha subjects, the compositions of postprandial Sf > 100 TRLs were TG poor and cholesterol and phospholipid enriched (p < 0.001) while the Sf 20-100 TRLs were enriched in cholesterol and phospholipid but relatively protein depleted (p = 0.002).(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic heterogeneity associated with high plasma triglyceride or low HDL cholesterol levels in men

To further understand the factors involved in the regulation of high plasma triglyceride (TG) or low plasma high density lipoprotein cholesterol (HDL-C) levels, three groups of male subjects (normal TG with low HDL-C levels, high TG with normal HDL-C levels, and high TG with low HDL-C levels) were compared with a sample of normolipemic men with normal TG and HDL-C plasma levels. Mean age was 34 years (range, 20-42 years), and none of the subjects had plasma TG levels > 4.0 mmol/l or familial hypercholesterolemia. Both groups of subjects with high TG levels had a higher body mass index, waist circumference, waist-to-hip circumferences ratio, and a higher ratio of abdominal to femoral adipose tissue areas as measured by computed tomography when compared with normolipemic control subjects. However, during an oral glucose tolerance test only high TG-low HDL-C men had fasting hyperinsulinemia and higher plasma insulin levels compared with normolipemic subjects. In addition, the high TG-low HDL-C group showed reduced HDL apoprotein (apo) A-I levels and a low HDL2-C/HDL3-C ratio. These changes were observed along with a nonsignificant trend for a lower plasma postheparin lipoprotein lipase activity. However, among subjects with high TG and normal HDL-C levels, no evidence of insulin resistance or of a reduction in postheparin lipoprotein lipase activity was observed, suggesting that the high plasma TG levels could be attributed to an increased production of apo B-containing lipoproteins, as high plasma apo B and low density lipoprotein (LDL)-apo B levels were observed in this group.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of intralipid-induced hypertriglyceridemia on plasma high-density lipoprotein metabolism in the cynomolgus monkey

Low plasma levels of high-density lipoprotein (HDL) and apolipoprotein (apo) A-I often accompany human hypertriglyceridemia. In an animal model of hypertriglyceridemia, the lipoprotein lipase (LPL)-inhibited cynomolgus monkey, we reported that plasma levels of apo A-I were decreased and the fractional catabolic rate (FCR) of HDL apo was increased. To explore whether hypertriglyceridemia alone would alter plasma apo A-I levels and catabolism, hypertriglyceridemia was produced by intravenous (IV) infusion of 20% Intralipid into female cynomolgus monkeys. Baseline plasma triglyceride (TG) levels averaged 106 mg/dL. With infusion of 200 mg/kg/h Intralipid TG, plasma TG levels peaked at 967 mg/dL (range, 413 to 1,069; n = 6). More prolonged or more severe hypertriglyceridemia caused serious complications in several monkeys. Despite the severe hypertriglyceridemia, HDL TG content, HDL apoproteins, and plasma apo A-I levels did not markedly change, suggesting that very little HDL remodeling had occurred. Kinetic studies of HDL protein and apo A-I were performed in four pairs of monkeys. The two tracers were removed from the plasma at identical rates. In five pairs of animals, apo A-I turnover during control and Intralipid-induced hypertriglyceridemia was not significantly different. We hypothesize that apo A-I FCR is a function of HDL composition. Because Intralipid infusion did not alter HDL composition to the same degree as did LPL inhibition, its effects on HDL apo catabolism were not apparent.

Low and high density lipoprotein metabolism in atherothrombotic brain infarction

BACKGROUND AND PURPOSE

Elevated low density lipoprotein and reduced high density lipoprotein cholesterol may increase the risk of atherothrombotic brain infarction, but the metabolic mechanisms accounting for this relation are poorly understood.

METHODS

The kinetic parameters of low density and high density lipoprotein were studied in nine subjects with atherothrombotic brain infarction or identifiable (by noninvasive testing) extracranial occlusive disease and in 12 control subjects. Autologous iodine-125-labeled lipoproteins were injected intravenously. Blood samples were drawn 10 minutes after injection and periodically thereafter for 10 days. Kinetic parameters were calculated from the decay curves.

RESULTS

The stroke-risk group showed significantly higher triglyceride (p less than 0.05), total cholesterol (p less than 0.02), and low density lipoprotein cholesterol (p less than 0.01). The fractional catabolic rate of low density lipoprotein was significantly lower (p less than 0.001) and the high density lipoprotein rate higher (p less than 0.02) in the stroke-risk group than in the control group. Regression analysis (using all subjects) of serum lipoproteins and their respective fractional catabolic rates correlated significantly (for low density lipoprotein, r = 0.684, p less than 0.001; for high density lipoprotein, r = 0.595, p less than 0.002). Mean percent stenosis showed a significant relation with triglyceride level (r = 0.678, p less than 0.01) and low density lipoprotein cholesterol (r = 0.535, p less than 0.02) but not with high density lipoprotein cholesterol. Mean percent stenosis also showed correlation with both fractional catabolic rate of low density lipoprotein (r = 0.667, p less than 0.002) and with serum high density lipoprotein levels (r = 0.504, p less than 0.02).

CONCLUSIONS

Our study provides insights into the role of altered low and high density lipoprotein metabolism in the pathogenesis of carotid stenosis. The statistically significant association of serum lipoprotein metabolic rates with carotid stenosis, rather than their respective serum concentrations, implies that metabolic parameters may be more important in predicting stroke risk.

In vivo metabolism of apolipoprotein A-I in a patient with homozygous familial hypercholesterolemia

Familial hypercholesterolemia (FH), caused by a defect in the low density lipoprotein (LDL) receptor, results in high plasma concentrations of LDL cholesterol due to both overproduction and delayed catabolism of LDL. FH is also associated with significantly lower levels of plasma high density lipoprotein cholesterol and apolipoprotein (apo) A-I in both heterozygous and homozygous patients. However, the metabolic basis of the hypoalphalipoproteinemia in FH has not been elucidated. We investigated the kinetics of apo A-I in a homozygous FH patient and two normal control subjects by using endogenous labeling with a stable isotopically labeled amino acid. Study subjects were administered a primed constant infusion of 13C6-phenylalanine for 12 hours. Apolipoproteins were isolated from plasma drawn at selected time points and analyzed for their isotopic enrichment by gas chromatography-mass spectrometry. The fractional catabolic rate of apo A-I in the FH subject was found to be substantially increased (0.38 day-1) compared with that of the normal subjects (mean, 0.26 day-1). In addition, the apo A-I production rate was decreased in the FH subject (6.5 mg/kg.day-1) compared with the normal subjects (mean, 11.1 mg/kg.day-1). In conclusion, the low levels of high density lipoprotein cholesterol and apo A-I in this homozygous FH patient are due to the combined metabolic defects of increased apo A-I catabolism and decreased apo A-I production.

Plasma apolipoproteins A-I, A-II, B, C-II, C-III, and E levels in young Kuwaiti male survivors of myocardial infarction

Plasm levels of lipid, lipoprotein, and apolipoprotein (APO) A-I, A-II, C-II, C-III, and E were analyzed in 180 young survivors of myocardial infarction (MI) ages 28 to 45 years and in 200 sex-and age-matched normal healthy subjects to assess the importance of apolipoprotein concentrations in comparison with lipoproteins in MI patients. In comparison with control subjects, MI patients showed marked increases in the following parameters: total cholesterol (TC), low density lipoproteins (LDL) cholesterol, triglycerides (TG), very low density (VLDL) triglycerides, and cholesterol, and LDL-TG, apo B, apo C-III and apo E. There were no significant changes in levels of HDL3; cholesterol, apo A-II and apo C-II in these patients compared with their controls. Levels of high density lipoprotein (HDL), HDL2 cholesterol, and plasma apo A-I were markedly decreased in the young MI survivors' group demonstrated that the better discriminating lipoproteins and apolipoproteins in MI patients in descending order were HDL2 cholesterol, apo C-III, apo B, apo A-I, VLDL triglycerides and HDL cholesterol discriminated between patients and controls. Results indicate that measurement of apo C-III, B and A-I beside HDL2 cholesterol were shown to be of potential use in differentiating normal controls from patients with MI.