Plasma triglycerides determine low density lipoprotein composition, physical properties, and cell-specific binding in cultured cells

Targeting structural flexibility in low density lipoprotein by integrating cryo-electron microscopy and high-speed atomic force microscopy

Low-density lipoprotein (LDL) plays a crucial role in cholesterol metabolism. Responsible for cholesterol transport from the liver to the organs, LDL accumulation in the arteries is a primary cause of cardiovascular diseases, such as atherosclerosis. This work focuses on the fundamental question of the LDL molecular structure, as well as the topology and molecular motions of apolipoprotein B-100 (apo B-100), which is addressed by single-particle cryo-electron microscopy (cryo-EM) and high-speed atomic force microscopy (HS-AFM). Our results suggest a revised model of the LDL core organization with respect to the cholesterol ester (CE) arrangement. In addition, a high-density region close to the flattened poles could be identified, likely enriched in free cholesterol. The most remarkable new details are two protrusions on the LDL surface, attributed to the protein apo B-100. HS-AFM adds the dimension of time and reveals for the first time a highly dynamic direct description of LDL, where we could follow large domain fluctuations of the protrusions in real time. To tackle the inherent flexibility and heterogeneity of LDL, the cryo-EM maps are further assessed by 3D variability analysis. Our study gives a detailed explanation how to approach the intrinsic flexibility of a complex system comprising lipids and protein.

Toward reliable low-density lipoprotein ultrastructure prediction in clinical conditions: A small-angle X-ray scattering study on individuals with normal and high triglyceride serum levels

Atherosclerosis is the main killer in the west and therefore a major health challenge today. Total serum cholesterol and lipoprotein concentrations, used as clinical markers, fail to predict the majority of cases, especially between the risk scale extremes, due to the high complexity in lipoprotein structure and composition. In particular, low-density lipoprotein (LDL) plays a key role in atherosclerosis development, with LDL size being a parameter considered for determining the risk for cardiovascular diseases. Determining LDL size and structural parameters is challenging to address experimentally under physiological-like conditions. This article describes the biochemistry and ultrastructure of normolipidemic and hypertriglyceridemic LDL fractions and subfractions using small-angle X-ray scattering. Our results conclude that LDL particles of hypertriglyceridemic compared to healthy individuals 1) have lower LDL core melting temperature, 2) have lower cholesteryl ester ordering in their core, 3) are smaller, rounder and more spherical below melting temperature, and 4) their protein-containing shell is thinner above melting temperature.

Long-term prognostic utility of low-density lipoprotein (LDL) triglyceride in real-world patients with coronary artery disease and diabetes or prediabetes

BACKGROUND

Recent guidelines highlighted the association between atherosclerosis and triglyceride-enriched lipoproteins in patients with impaired glucose metabolism. However, evidence from prospective studies for long-term prognostic utility of low-density lipoprotein triglyceride (LDL-TG) in real-world patients with prediabetes (Pre-DM) or diabetes mellitus (DM) and coronary artery disease (CAD) is currently not available. The aim of the present study was to evaluate the impact of LDL-TG on major adverse cardiovascular events (MACEs) in patients with stable CAD under different glucose metabolism status.

METHODS

A total of 4381 patients with CAD were consecutively enrolled and plasma LDL-TG level was measured by an automated homogeneous assay. They were categorized according to both status of glucose metabolism [DM, Pre-DM, normal glycaemia regulation (NGR)] and tertiles of LDL-TG. All subjects were followed up for the occurrence of MACEs.

RESULTS

During a median of 5.1 (interquartile range 3.9 to 5.9) years' follow-up, 507 (11.6%) MACEs occurred. Cubic spline models showed a significant association between LDL-TG and MACEs in DM and Pre-DM but not in NGR. When the combined effect of elevated LDL-TG and glucose disorders was considered for risk stratification, the medium tertile of LDL-TG plus DM, and the highest tertile of LDL-TG plus Pre-DM or plus DM subgroups were associated with significantly higher risk of MACEs after adjustment of confounders including triglyceride [hazard ratios (95% confidence intervals): 1.843 (1.149-2.955), 1.828 (1.165-2.867), 2.212 (1.396-3.507), all p < 0.05]. Moreover, adding LDL-TG into the original model increased the C-statistic from 0.687 to 0.704 (∆C-statistic = 0.016, p = 0.028) and from 0.734 to 0.749 (∆C-statistic = 0.014, p = 0.002) in Pre-DM and DM, respectively.

CONCLUSIONS

In this longitudinal cohort study on real-world practice, higher LDL-TG was associated with worse outcomes among Pre-DM and DM patients with stable CAD.

Biochemical markers as diagnostic/prognostic indicators for ischemic disease

Objective

The use of a biomarker was extremely useful in clinical emergencies such as stroke to aid in triage and early management of cases. The diagnostic accuracy of laboratory biomarkers is run to approve the identification of easy, cheap and fast tests associated with cerebral ischemia and intracranial hemorrhage. The present study was designed to screen serum enolase activity, activities of CK-BB, LDH and lipid profile in patients with ischemic or related diseases as good diagnostic/ prognostic indicator for ischemic diseases.

Methods

Sixty male subjects in the age range of (45 ±2years) were divided into four groups each with 15 participants: Group (I) normal . Group (II) patients recently diagnosed as ischemic disease; Group (III) hypertensive patients and Group (IV); diabetic patients enolase activity (p<0.001) and CK-BB (p<0.01) in ischemic and hypertensive patients compared with control and diabetic groups. LDH level was significantly elevated in ischemic, hypertensive and diabetic patients compared with controls (p<0.001). The cut -off value for serum enolase was 62.5 nmol/l showing 90% sensitivity and 93% specificity for differentiation of ischemic disease. Positive correlations were observed between serum enolase (r = 0.56), and CK-BB (r = 0.53).

Conclusion

Serum enolase can be considered as a more sensitive and specific marker and used as a sensitive diagnostic or prognostic marker for ischemic related diseases.

High Hydrostatic Pressure Induces a Lipid Phase Transition and Molecular Rearrangements in Low-Density Lipoprotein Nanoparticles

Low-density lipoproteins (LDL) are natural lipid transporter in human plasma whose chemically modified forms contribute to the progression of atherosclerosis and cardiovascular diseases accounting for a vast majority of deaths in westernized civilizations. For the development of new treatment strategies, it is important to have a detailed picture of LDL nanoparticles on a molecular basis. Through the combination of X-ray and neutron small-angle scattering (SAS) techniques with high hydrostatic pressure (HHP) this study describes structural features of normolipidemic, triglyceride-rich and oxidized forms of LDL. Due to the different scattering contrasts for X-rays and neutrons, information on the effects of HHP on the internal structure determined by lipid rearrangements and changes in particle shape becomes accessible. Independent pressure and temperature variations provoke a phase transition in the lipid core domain. With increasing pressure an inter-related anisotropic deformation and flattening of the particle are induced. All LDL nanoparticles maintain their structural integrity even at 3000 bar and show a reversible response toward pressure variations. The present work depicts the complementarity of pressure and temperature as independent thermodynamic parameters and introduces HHP as a tool to study molecular assembling and interaction processes in distinct lipoprotein particles in a nondestructive manner.

Hypolipidemic treatment of heterozygous familial hypercholesterolemia: a lifelong challenge

In familial hypercholesterolemia, a defect in low-density lipoprotein receptors causes lifelong two- to threefold elevations in serum low-density lipoprotein-cholesterol levels. This leads to early atherosclerotic changes in infancy. Lifelong hypolipidemic treatment that can be started at a young age is thus greatly needed. Early diagnosis of familial hypercholesterolemia is important, and improved DNA tests for low-density lipoprotein receptor mutations have made it possible to carry out diagnosis at birth. A low saturated-fat, low cholesterol diet can be safely started at 7 months of age. This can be accompanied by dietary stanol esters from 2 years of age. At the age of 10, statin treatment can be safely started. In adults, more aggressive hypolipidemic treatment is required in order to reach the treatment goal for serum low-density lipoprotein-cholesterol levels less than 2.5 mmol/l. This can be achieved by using high doses of statin, or preferably by combining a statin with resin or ezetimibe (Zeita), Merck and Shering-Plough Pharmaceuticals). Once started, treatment of familial hypercholesterolemia is lifelong.

Genetics and cardiovascular system: influence of human genetic variants on vascular function

Candidate gene association studies in cardiovascular diseases have provided evidence on the molecular basis of phenotypic differences between individuals. The comprehension of how inherited genetic variants are able to affect protein functions has increased the knowledge of how genes interact with environment in order to modulate a particular phenotype. Although it is known that the human genome contains more than 10 million SNPs, only a minor part of them are supposed to be functional. A causative SNP in a particular gene may confer a small to moderate effect in complex phenotypes, such as functions important to cardiovascular homeostasis. This paper is a selective review of the literature on the evidence for interactions between vascular function and naturally occurring genetic variants in endothelial nitric oxide synthase (eNOS) and beta-2 adrenergic receptor (ADRB2), two genes among those influencing vascular phenotype and examples for which there is a strong evidence base. eNOS and ADRB2 will be characterized, as well as the mechanisms by which the enzyme and the receptor work to control vascular responses will be described. Understanding the molecular mechanisms underlying gene-mediated vascular function and their modification by genetic variants is expected to result in a better comprehension about individual's phenotypic differences.

A high-fat meal impairs muscle vasodilatation response to mental stress in humans with Glu27 beta2-adrenoceptor polymorphism

Background

Forearm blood flow responses during mental stress are greater in individuals homozygous for the Glu27 allele. A high-fat meal is associated with impaired endothelium-dependent dilatation. We investigated the impact of high-fat ingestion on the muscle vasodilatory responses during mental stress in individuals with the Glu27 allele and those with the Gln27 allele of the β₂-adrenoceptor gene.

Methods

A total of 162 preselected individuals were genotyped for the Glu27Gln β₂-adrenoceptor polymorphism. Twenty-four individuals participated in the study. Fourteen were homozygous for the Gln27 allele (Gln27Gln, 40 ± 2 years; 64 ± 2 kg), and 10 were homozygous for the Glu27 allele (Glu27Glu, 40 ± 3 years; 65 ± 3 kg). Forearm blood flow was evaluated by venous occlusion plethysmography before and after ingestion of 62 g of fat.

Results

The high-fat meal caused no changes in baseline forearm vascular conductance (FVC, 2.2 ± 0.1 vs. 2.4 ± 0.2; P = 0.27, respectively), but reduced FVC responses to mental stress (1.5 ± 0.2 vs. 0.8 ± 0.2 units; P = 0.04). When volunteers were divided according to their genotypes, baseline FVC was not different between groups (Glu27Glu = 2.4 ± 0.1 vs. Gln27Gln = 2.1 ± 0.1 units; P = 0.08), but it was significantly greater in Glu27Glu individuals during mental stress (1.9 ± 0.4 vs. 1.0 ± 0.3 units; P = 0.04). High-fat intake eliminated the difference in FVC responses between Glu27Glu and Gln27Gln individuals (FVC, 1.3 ± 0.4 vs. 1.2 ± 0.4; P = 0.66, respectively).

Conclusion

These findings demonstrate that a high-fat meal impairs muscle vasodilatation responses to mental stress in humans. However, this reduction can be attributed to the presence of the homozygous Glu27 allele of the β₂-adrenoceptor gene.

Model of human low-density lipoprotein and bound receptor based on cryoEM

Human plasma low-density lipoproteins (LDL), a risk factor for cardiovascular disease, transfer cholesterol from plasma to liver cells via the LDL receptor (LDLr). Here, we report the structures of LDL and its complex with the LDL receptor extracellular domain (LDL.LDLr) at extracellular pH determined by cryoEM. Difference imaging between LDL.LDLr and LDL localizes the site of LDLr bound to its ligand. The structural features revealed from the cryoEM map lead to a juxtaposed stacking model of cholesteryl esters (CEs). High density in the outer shell identifies protein-rich regions that can be accounted for by a single apolipoprotein (apo B-100, 500 kDa) leading to a model for the distribution of its alpha-helix and beta-sheet rich domains across the surface. The structural relationship between the apo B-100 and CEs appears to dictate the structural stability and function of normal LDL.

CETP and oxidized LDL levels increase in dyslipidemic subjects

OBJECTIVES

To explore the possible associations among cholesteryl ester transfer protein (CETP), contents of lipids in low-density lipoprotein (LDL), and in vivo oxidized LDL (Ox-LDL).

DESIGN AND METHODS

CETP and Ox-LDL were both detected by ELISA. Their levels and the lipid contents of LDL were investigated in 200 subjects with various dyslipidaemias.

RESULTS

Compared to the control, CETP levels were significantly increased in subjects with mixed hyperlipidaemia and hypercholesterolaemic. Ox-LDL levels were only significantly increased in mixed hyperlipidaemia subjects. Triacyglycerols to cholesterol ratio in LDL was significantly increased in various dyslipidaemias subjects, of which, hypertriglyceridaemic subjects exhibited the most significant change, while hypercholesterolaemic subjects the least. Multiple linear regression analysis showed that total cholesterol and triacyglycerols levels in very low-density lipoprotein were significantly related with CETP (R(2)=0.066), and triacyglycerols and total cholesterol levels were significantly related with Ox-LDL (R(2)=0.094), respectively.

CONCLUSIONS

High CETP promotes the transport of lipids among lipoproteins, which changed the lipid composition of LDL, resulting in the increase of in vivo Ox-LDL level, and subsequently contributing to the atherogenic process in dyslipidaemias subjects.

Electronegative LDLs from familial hypercholesterolemic patients are physicochemically heterogeneous but uniformly proapoptotic

A highly electronegative fraction of human plasma LDLs, designated L5, has distinctive biological activity that includes induction of apoptosis in bovine aortic endothelial cells (BAECs). This study was performed to identify a relationship between LDL density, electronegativity, and biological activity, namely, the induction of apoptosis in BAECs. Plasma LDLs from normolipidemic subjects and homozygotic familial hypercholesterolemia subjects were separated into five subfractions, with increasing electronegativity from L1 to L5, and into seven subfractions according to increasing density, D1 to D7. L1 to L5 were also separated according to density, and D1 to D7 were separated according to charge. The density profiles of L1 to L5 were similar (maximum density = 1.030 +/- 0.002 g/ml). Induction of apoptosis by all seven density subfractions was confined to the highly electronegative fraction, L5, and within each density subfraction the magnitude of apoptosis correlated with the L5 content. Electronegative LDL is heterogeneous with respect to density and composition, and induction of apoptosis is more strongly associated with LDL electronegativity than with LDL size or density.

Metabolism of phytol to phytanic acid in the mouse, and the role of PPARα in its regulation

Phytol, a branched-chain fatty alcohol, is the naturally occurring precursor of phytanic and pristanic acid, branched-chain fatty acids that are both ligands for the nuclear hormone receptor peroxisome proliferator-activated receptor alpha (PPARalpha). To investigate the metabolism of phytol and the role of PPARalpha in its regulation, wild-type and PPARalpha knockout (PPARalpha-/-) mice were fed a phytol-enriched diet or, for comparison, a diet enriched with Wy-14,643, a synthetic PPARalpha agonist. After the phytol-enriched diet, phytol could only be detected in small intestine, the site of uptake, and liver. Upon longer duration of the diet, the level of the (E)-isomer of phytol increased significantly in the liver of PPARalpha-/- mice compared with wild-type mice. Activity measurements of the enzymes involved in phytol metabolism showed that treatment with a PPARalpha agonist resulted in a PPARalpha-dependent induction of at least two steps of the phytol degradation pathway in liver. Furthermore, the enzymes involved showed a higher activity toward the (E)-isomer than the (Z)-isomer of their respective substrates, indicating a stereospecificity toward the metabolism of (E)-phytol. In conclusion, the results described here show that the conversion of phytol to phytanic acid is regulated via PPARalpha and is specific for the breakdown of (E)-phytol.

Postprandial hyperglycaemia and vascular damage--the benefits of acarbose

Traditional risk factors do not fully explain the increased risk of cardiovascular disease (CVD) in diabetes. Epidemiology shows that hyperglycaemia is a continuous CVD risk factor and that two-hour postprandial glucose levels are more strongly associated with CVD than fasting glucose. Good glycaemic control is proven to reduce the risk of microvascular complications, but equivalent evidence for CVD risk reduction is lacking. However, in the Study to Prevent Non-Insulin Dependent Diabetes Mellitus (STOP-NIDDM), acarbose reduced the risk of diabetes in those with impaired glucose tolerance by 36% (p=0.0017) and the risk of any cardiovascular event by 49% (p=0.0326) versus placebo. Furthermore, a meta-analysis of trials of acarbose in patients with type 2 diabetes suggests a significant reduction of CVD events. This review examines evidence that postprandial hyperglycaemia plays a major role in vascular damage, particularly through non-traditional risk factors such as oxidative stress and subclinical inflammation, and how acarbose may prevent this damage.

Lipoproteins: When size really matters

The field of nanoscience is extending the applications of physics, chemistry and biology into previously unapproached infinitesimal length scales. Understanding the behavior and manipulating the positions and properties of single atoms and molecules hold great potential to improve areas of science as disparate as medicine and computation, and communication and orbiting satellites. Yet, in the race to develop novel, previously unavailable nanoparticles, there is an opportunity for scientists in this field to digress and to apply their growing understanding of nanoscience and the tools of nanotechnology to one of the most pressing problems in all of human biology-diseases related to lipoproteins. Although not appreciated outside the field of lipoprotein biology, variations in the compositions, structures and properties of these nanoscale-sized, blood-borne particles are responsible for most of the variations in health, morbidity and mortality in the Western world. If the lipoproteins could be understood at the nanometer length scale with precise details of their structures and functions, scientists could understand a wide range of perplexing physiological processes and also address the dysfunctions in normal lipoprotein biology that lead to such diseases as hypercholesterolemia, heart disease, stroke and neurodegenerative diseases. Furthermore, if the capabilities of nanoscience to assemble and manipulate nanometer-sized particles could be recruited to studies of lipoproteins, these biological particles would provide a new dimension to therapeutic agents, and these natural particles could be designed to carry out many specialized beneficial tasks.

Postprandial, but not postabsorptive low-density lipoproteins increase the expression of intercellular adhesion molecule-1 in human aortic endothelial cells

The magnitude of postprandial lipemia has been identified as independent risk factor for the development of coronary artery disease. To test the effect of postprandial versus postabsorptive low-density lipoproteins (LDL) on the expression of adhesion molecules, LDL were isolated from healthy subjects before and 4h after ingestion of a standardized fatty test meal. We used flow cytometry and Northern blotting to quantify cell adhesion molecules in human aortic endothelial cells (HAEC). The adherence of leukocytes to HAEC was analyzed using a monocyte adhesion assay. Incubation of HAEC with postprandial, but not postabsorptive LDL induced a two-fold increase in the surface expression of intercellular adhesion molecule-1 (ICAM-1), but not of E-selectin or vascular cell adhesion molecule-1. In addition, increased amounts of ICAM-1 transcripts were found in HAEC treated with postprandial LDL. The adhesion of monocytes to HAEC was enhanced after pretreatment with postprandial, but not with postabsorptive LDL. We conclude that postprandial, but not postabsorptive LDL increase the surface expression of ICAM-1 in HAEC apparently by de novo protein synthesis leading to increased adhesion of monocytes. The upregulation of ICAM-1 by postprandial LDL may explain part of the proatherogenic effect of high postprandial lipemia.

Apolipoprotein B is conformationally flexible but anchored at a triolein/water interface: a possible model for lipoprotein surfaces

Apolipoprotein B (apoB) is one of a unique group of proteins that form and bind to fat droplets, stabilize the emulsified fat, and direct their metabolism. ApoB, secreted on lipoproteins (emulsions), remains bound during lipid metabolism yet exhibits conformational flexibility. It has amphipathic beta-strand (AbetaS)-rich domains and amphipathic alpha-helix (AalphaH)-rich domains. We showed that two consensus AbetaS peptides of apoB bound strongly to hydrophobic interfaces [triolein/water (TO/W) and dodecane/water], were elastic, and were not pushed off the interface when the surface was compressed. In contrast, an AalphaH peptide modeling helical parts of apoB was forced off the TO/W interface by compression and readsorbed when the interface was expanded. In this report, the surface behavior of apoB-100 was studied at the TO/W interface. Solubilized apoB lowered the interfacial tension of TO/W in a concentration-dependent fashion. At equilibrium tension, if the surface was compressed, part of apoB was pushed off but quickly readsorbed when the surface was expanded. Even when the surface area was compressed by approximately 55%, part of the apoB molecule remained bound. The maximum surface pressure that apoB could withstand without being partially ejected was 13 mN/m. ApoB showed high elasticity at the TO/W interface. Based on studies of the consensus AbetaS and AalphaH peptides, we suggest that AbetaSs anchor apoB and are its nonexchangeable motif, whereas its conformational flexibility arises from both the elastic nature of the AbetaS and the ability of AalphaH domains of the molecule to desorb and readsorb rapidly in response to surface pressure changes.

Lipid composition influences the shape of human low density lipoprotein in vitreous ice

Earlier cryo-electron microscopic studies have indicated that the normal low density lipoprotein (N-LDL) has a discoid shape when its core is in the liquid-crystalline state. In the present study, we investigated whether the shape of LDL depends on the physical state and/or the lipid composition of the lipoprotein core. Using a custom-built freezing device, we vitrified NLDL samples from either above or below the phase-transition temperature of the core (42 and 24 degrees C, respectively). Cryo-electron microscopy revealed no differences between these samples and indicated a discoid shape of the N-LDL particle. In contrast, TG-enriched LDL (T-LDL) did not have discoid features and appeared to be quasi-spherical in preparations that were vitrified from either 42 or 24 degrees C. These results suggest that the shape of NLDL is discoid, regardless of the physical state of its core, whereas T-LDL is more spherical. Aspects that may influence the shape of LDL are discussed.

Impaired Binding Affinity of Electronegative Low-Density Lipoprotein (LDL) to the LDL Receptor Is Related to Nonesterified Fatty Acids and Lysophosphatidylcholine Content†

The binding characteristics of electropositive [LDL(+)] and electronegative LDL [LDL(-)] subfractions to the LDL receptor (LDLr) were studied. Saturation kinetic studies in cultured human fibroblasts demonstrated that LDL(-) from normolipemic (NL) and familial hypercholesterolemic (FH) subjects had lower binding affinity than their respective LDL(+) fractions (P < 0.05), as indicated by higher dissociation constant (K(D)) values. FH-LDL(+) also showed lower binding affinity (P < 0.05) than NL-LDL(+) (K(D), sorted from lower to higher affinity: NL-LDL(-), 33.0 +/- 24.4 nM; FH-LDL(-), 24.4 +/- 7.1 nM; FH-LDL(+), 16.6 +/- 7.0 nM; NL-LDL(+), 10.9 +/- 5.7 nM). These results were confirmed by binding displacement studies. The impaired affinity binding of LDL(-) could be attributed to altered secondary and tertiary structure of apolipoprotein B, but circular dichroism (CD) and tryptophan fluorescence (TrpF) studies revealed no structural differences between LDL(+) and LDL(-). To ascertain the role of increased nonesterified fatty acids (NEFA) and lysophosphatidylcholine (LPC) content in LDL(-), LDL(+) was enriched in NEFA or hydrolyzed with secretory phospholipase A(2). Modification of LDL gradually decreased the affinity to LDLr in parallel to the increasing content of NEFA and/or LPC. Modified LDLs with a NEFA content similar to that of LDL(-) displayed similar affinity. ApoB structure studies of modified LDLs by CD and TrpF showed no difference compared to LDL(+) or LDL(-). Our results indicate that NEFA loading or phospholipase A(2) lipolysis of LDL leads to changes that affect the affinity of LDL to LDLr with no major effect on apoB structure. Impaired affinity to the LDLr shown by LDL(-) is related to NEFA and/or LPC content rather than to structural differences in apolipoprotein B.

Evidence that hepatic lipase deficiency in humans is not associated with proatherogenic changes in HDL composition and metabolism

The aim of the present study was to characterize the composition and metabolism of HDL in subjects with complete hepatic lipase (HL) deficiency. Analyses were carried out in three complete and three partial HL-deficient subjects as well as in eight normotriglyceridemic (NTG) and two hypertriglyceridemic controls. Complete HL deficiency was associated with hypertriglyceridemia and with a 3.5-fold increase in HDL-triglyceride (TG) levels. The in vivo kinetics of apolipoprotein A-I (apoA-I) and apoA-II (d < 1.25 g/l) were studied in the fasted state using a primed-constant infusion of l-(5,5,5-D3)leucine for 12 h. Complete HL deficiency was associated with a reduced fractional catabolic rate of apoA-I in the HL-deficient female proband (-47%) and in her two brothers (-21%) compared with gender- and TG-matched controls. Total plasma and HDL from complete HL-deficient patients were able to mediate normal cholesterol efflux from human skin fibroblasts labeled with [3H]cholesterol. Complete HL deficiency was also associated with normal levels of prebeta-migrating apoA-I-containing HDL separated by two-dimensional gel electrophoresis and with an accumulation of large HDL particles compared with NTG controls. These results suggest that HL activity is important for adequate HDL metabolism, although its presence may not be necessary for normal HDL-mediated reverse cholesterol transport.

Molecular Mechanism for Changes in Proteoglycan Binding on Compositional Changes of the Core and the Surface of Low-Density Lipoprotein–Containing Human Apolipoprotein B100

OBJECTIVE

The aim of this study was to investigate the molecular mechanism for changes in proteoglycan binding and LDL receptor affinity on two compositional changes in LDL that have been associated with atherosclerosis: cholesterol enrichment of the core and modification by secretory group IIA phospholipase A2 (sPLA2) of the surface.

METHODS AND RESULTS

Transgenic mice expressing recombinant apolipoprotein (apo) B and sPLA2 were generated. Recombinant LDL were isolated and tested for their proteoglycan and LDL receptor-binding activity. The results show site A (residues 3148-3158) in apoB100 becomes functional in sPLA2-modified LDL and that site A acts cooperatively with site B (residues 3359-3369), the primary proteoglycan-binding site in native LDL, in the binding of sPLA2-modified LDL to proteoglycans. Our results also show that cholesterol enrichment of LDL is associated with increased affinity for proteoglycans and for the LDL receptor. This mechanism is likely mediated by a conformational change of site B and is independent of site A in apoB100.

CONCLUSIONS

Site A in apoB100 becomes functional in sPLA2-modified LDL and acts cooperatively with site B resulting in increased proteoglycan-binding activity. The increased binding for proteoglycans of cholesterol-enriched LDL is solely dependent on site B.

Pathophysiology of dyslipidemia and increased cardiovascular risk in HIV lipodystrophy: a model of 'systemic steatosis'

PURPOSE OF REVIEW

This review addresses a syndrome of dyslipidemia and lipodystrophy that has emerged in HIV-infected patients receiving highly active antiretroviral therapy (HAART). The term 'HIV/HAART associated dyslipidemic lipodystrophy (HADL)' describes this syndrome. Although HAART increases patient survival rates, their increased longevity and dyslipidemias place them at risk for cardiovascular disease. Identification of rationally based therapies requires an understanding of the mechanistic basis of HADL.

RECENT FINDINGS

A case definition for HIV lipodystrophy, based on age, gender, duration of HIV disease, serum HDL cholesterol and anthropometry, provides high diagnostic sensitivity and specificity. The dyslipidemias, mainly hypercholesterolemia, hypertriglyceridemia and low-plasma HDL cholesterol, among HIV-infected patients in the pre- and post-HAART eras are summarized. Clinical studies of HADL patients show increased lipolysis, which increases free fatty acid transfer to liver for incorporation into lipoprotein triglycerides that are secreted, and to skeletal muscle where they impair normal insulin signaling. A model of HADL that includes preferential lipolysis in femoral-gluteal fat depots is presented. Relevant therapies include those that inhibit lipolysis (niacin) or increase hepatic fatty acid oxidation (fibrates).

SUMMARY

HADL is one of several disorders characterized by dyslipidemia, insulin resistance, and lipodystrophy. The relative acuteness of HADL should facilitate identification of the sequence of metabolic changes that gives rise to the syndrome. Current evidence suggests that deranged energy storage in femoral-gluteal and other peripheral sites is important; the molecular details for the derangement are unknown but are under scrutiny by many investigators.

Structure of Triglyceride-Rich Human Low-Density Lipoproteins According to Cryoelectron Microscopy

Low-density lipoprotein (LDL) particles from normolipidemic individuals contain a cholesteryl ester-rich core that undergoes a thermal transition from a liquid crystalline to an isotropic liquid phase between 20 and 35 degrees C. LDL from hypertriglyceridemic patients or prepared in vitro by the exchange of very low-density lipoprotein for LDL cholesteryl esters is triglyceride-rich, does not have a thermal transition above 0 degrees C, and exhibits impaired binding to the LDL receptor on normal human skin fibroblasts. Cryoelectron microscopy of LDL quick-frozen from 10 (core-frozen) and 40 degrees C (core-melted) revealed ellipsoidal particles with internal striations and round particles devoid of striations, respectively. Cryoelectron microscopy of triglyceride-rich LDL prepared in vitro revealed particles similar to the core-melted normolipidemic LDL, i.e., round particles without striations. These data suggest that the LDL core in the liquid crystalline phase is characterized by the appearance of striations, whereas LDL with a core that is an isotropic liquid lacks striations. It is suggested that freezing the LDL core into a liquid crystalline phase imposes structural constraints that force LDL from a sphere without partitions to an ellipsoid with partitions. We further suggest that the striation-defined lamellae are a structural feature of a liquid crystalline neutral lipid core that is a determinant of normal binding to the LDL receptor and that conversion of the neutral lipid core of LDL to the isotropic liquid phase via an increase in the temperature or via the addition of triglyceride partially ablates the receptor binding determinants on the LDL surface. This effect is likely achieved through changes in the conformation of apo-B-100. These data suggest that the physical state of the LDL core determines particle shape, surface structure, and metabolic fate.

Improved cardiovascular risk profile and renal function in renal transplant patients after randomized conversion from cyclosporine to tacrolimus

Cyclosporine is considered to contribute to the high cardiovascular morbidity and mortality in patients after renal transplantation. Tacrolimus may be more favorable in this respect, but controlled data are scarce. In this prospective randomized study in 124 stable renal transplant patients, the effects of conversion from cyclosporine to tacrolimus on cardiovascular risk factors and renal function were investigated. Follow-up was 6 mo. Statistical analysis was performed by ANOVA for repeated measurements. The serum creatinine level decreased from 137 +/- 30 micromol/L to 131 +/- 29 micromol/L (P < 0.01). Three months after conversion from cyclosporine to tacrolimus, mean BP significantly decreased from 104 +/- 13 to 99 +/- 12 mmHg (P < 0.001). Serum LDL cholesterol decreased from 3.48 +/- 0.80 to 3.11 +/- 0.74 mmol/L (P < 0.001,) and serum apolipoprotein B decreased from 1018 +/- 189 to 935 +/- 174 mg/L (P < 0.001). Serum triglycerides decreased from 2.11 +/- 1.12 to 1.72 +/- 0.94 mmol/L (P < 0.001). In addition, both rate and extent of LDL oxidation were reduced. The fibrinogen level decreased from 3638 +/- 857 to 3417 +/- 751 mg/L (P < 0.05). Plasma homocysteine concentration did not change. Three months after conversion, plasma fasting glucose level temporarily increased from 5.4 +/- 1.3 mmol/L to 5.8 +/- 1.9 mmol/L (P < 0.05). Conversion to tacrolimus resulted in a significant reduction of the Framingham risk score. In conclusion, conversion from cyclosporine to tacrolimus in stable renal transplant patients has a beneficial effect on renal function, BP, serum concentration and atherogenic properties of serum lipids, and fibrinogen.

Isolation, characterization, and functional assessment of oxidatively modified subfractions of circulating low-density lipoproteins

OBJECTIVE

Current evidence suggests that oxidatively modified human plasma low-density lipoproteins (ox-LDLs) are proatherogenic and cytotoxic to endothelial and vascular smooth muscle cells. The present study describes a method using ion-exchange chromatography that is capable of large-scale subfractionation of LDL for adequate analyses of composition or bioactivities.

METHODS AND RESULTS

LDLs from normolipidemic (N-LDL) and homozygous familial hypercholesterolemic (FH-LDL) subjects were separated into 5 subfractions (L1 through L5) by high-capacity ion-exchange chromatography. The most strongly retained fraction from FH subjects, FH-L5, suppressed DNA synthesis in cultured bovine aortic endothelial cells and stimulated mononuclear cell adhesion to cultured endothelial cells under flow conditions in vitro. L5, which represented 1.1+/-0.2% and 3.7+/-1.7% of the LDL from N-LDL and FH-LDL, respectively, was more triglyceride-rich (17% versus 5%) and cholesteryl ester-poor (23% versus 33%) than were L1 through L4. Electrophoretic mobilities on agarose gels increased from L1 to L5. According to SDS-PAGE, apolipoprotein B-100 in N-LDL fractions L1 through L5 appeared as a single approximately 500-kDa band. In contrast, the fractions isolated from FH-LDL showed substantial fragmentation of the apolipoprotein B-100, including bands between 200 and 116 kDa. Competitive ELISA analyses using a malondialdehyde-specific monoclonal antibody against Cu2+ ox-LDL suggest that FH-L5 is malondialdehyde-modified.

CONCLUSIONS

Relative to N-LDL, FH-LDL contains higher concentrations of a fraction, L5, that exhibits distinctive physicochemical properties and biological activities that may contribute to initiation and progression of atherogenesis in vivo.

Immunochemical evidence that human apoB differs when expressed in rodent versus human cells

LDL from human apolipoprotein B-100 (apoB-100) transgenic (HuBTg+/+) mice contains more triglyceride than LDL from normolipidemic subjects. To obtain novel monoclonal antibody (MAb) probes of apoB conformation, we generated hybridomas from HuBTg+/+ that had been immunized with LDL isolated from human plasma. One apoE-specific and four anti-apoB-100-specific hybridomas were identified. Two MAbs, 2E1 and 3D11, recognized an epitope in the amino-terminal 689 residues of apoB in native apoB-containing lipoproteins (LpBs) from human plasma or from the supernatant of human hepatoma HepG2 cells, but did not react with LpB from HuBTg+/+ mice or LpB secreted by human apoB-100-transfected rat McArdle 7777 hepatoma cells. 2E1 reacted weakly and 3D11 reacted strongly with apoB from HuBTg+/+ mice after SDS-PAGE. The lack of expression of the 2E1 and 3D11 epitopes on native LpB from HuBTg+/+ mice did not solely reflect the abnormal lipid composition of murine LpB. Both epitopes were detected in all human plasma samples tested and in all human plasma LpB classes. Therefore, human apoB expressed by rodent hepatocytes or hepatoma cells appears to adopt a different conformation or undergoes different posttranslational modification than apoB expressed in human hepatocytes or hepatoma cells.

Insulin improves fasting and postprandial lipemia in type 2 diabetes

BACKGROUND: The purpose of the investigation presented here was to study the effects of insulin therapy in type 2 diabetes mellitus (type 2 DM) not only on glycemic control but also on other components of the metabolic syndrome, including lipid metabolism, blood pressure, and body weight. METHODS: Twelve patients with type 2 DM were studied before and after replacement of sulphonylurea treatment with insulin for 4 months. RESULTS: Insulin therapy resulted in a significant decrease in fasting glucose levels by 26%; glycated hemoglobin decreased by 17% and fructosamine values by 19%. With insulin treatment, fasting plasma triglyceride levels decreased by 28% and total HDL cholesterol and HDL(3) cholesterol increased by 17 and 11%, respectively. Low-density lipoprotein (LDL) cholesterol showed no significant change. The magnitude of postprandial lipemia after ingestion of a standard fatty meal decreased by 38%. Insulin treatment was also accompanied by a 21% increase in lipoprotein lipase (LPL) activity in postheparin plasma and by a 20% increase in cholesteryl ester transfer protein (CETP) activity. Hepatic lipase activity was not changed significantly with insulin. Mean BMI decreased from 28.5+/-4.2 to 28.0+/-3.1 kg/m(2) (P=0.02), which is in keeping with the finding that peripheral insulin levels did not increase and which can be explained by the fact that the insulin regimen was combined with dietary counseling. Accordingly, blood pressure showed no significant change. CONCLUSION: Our study demonstrates that judicious replacement of sulfonylurea treatment with insulin therapy, together with dietary counseling, can result in a simultaneous improvement in the major stigmata of the metabolic syndrome, i.e. a significant improvement in glycemic control and lipid metabolism without unfavorable effects on body weight and blood pressure.

Endothelial function in the post-prandial state

In recent years the endothelium has emerged as an important protective barrier against the development of atherosclerosis. Endothelial dysfunction, often characterised by impaired endothelium-dependent vasodilation, is generally considered to be an early marker of cardiovascular disease. Recently, it has been noted that the post-prandial state is associated with impaired endothelium-dependent vasodilation. Since man lives in a post-prandial state most of the day, prolonged exposure to dietary factors which impair endothelial function or delayed post-prandial recovery of the endothelium could negatively affect the cardiovascular risk profile. This review will focus on post-prandial endothelial function characterised by endothelium-dependent changes in vasomotion.

Evaluation of coronary risk factors in patients with heterozygous familial hypercholesterolemia

Age at onset of clinically manifested coronary artery disease (CAD) varies widely among patients with familial hypercholesterolemia (FH). A number of factors in addition to high low-density lipoprotein cholesterol (LDL) have been suggested as predictors of risk among patients with FH, but a comprehensive examination of their utility is lacking. We therefore measured plasma lipids, carotid intima-medial thickness, and a variety of coronary risk factors in 262 patients with FH > or = 30 years old (68 of whom had premature CAD). Age (p < 0.0001) and gender were the most important determinants of premature CAD risk, with men having 5.64 times the risk of women (p < 0.0001). In addition, cigarette smoking (odds ratio [OR] 2.71, p = 0.026), smaller LDL as determined by the LDL cholesterol/LDL apolipoprotein B ratio (OR 2.60, p = 0.014), and white blood cell count (p = 0.014) were also statistically significant risk factors. Lipoprotein(a) and the presence of xanthoma were associated with risk only in very early coronary cases. After correction for age, carotid intima-media thickness was not associated with CAD risk. Insulin, fibrinogen, homocysteine, plasma C-reactive protein, and the angiotensin-converting enzyme insertion/deletion polymorphism were unrelated to risk in this cohort. These results provide little justification for extensive investigation of risk factors among patients with FH, at least for the risk factors examined here. Rather, the inherent high LDL cholesterol of these patients should be the focus of preventive efforts. The novel finding of increased risk with smaller LDL may prove useful but needs further confirmation.

Structure of low density lipoprotein (LDL) particles: basis for understanding molecular changes in modified LDL

Low density lipoprotein (LDL) particles are the major cholesterol carriers in circulation and their physiological function is to carry cholesterol to the cells. In the process of atherogenesis these particles are modified and they accumulate in the arterial wall. Although the composition and overall structure of the LDL particles is well known, the fundamental molecular interactions and their impact on the structure of LDL particles are not well understood. Here, the existing pieces of structural information on LDL particles are combined with computer models of the individual molecular components to give a detailed structural model and visualization of the particles. Strong evidence is presented in favor of interactions between LDL lipid constituents that lead to specific domain formation in the particles. A new three-layer model, which divides the LDL particle into outer surface, interfacial layer, and core, and which is capable of explaining some seemingly contradictory interpretations of molecular interactions in LDL particles, is also presented. A new molecular interaction model for the beta-sheet structure and phosphatidylcholine headgroups is introduced and an overall view of the tertiary structure of apolipoprotein B-100 in the LDL particles is presented. This structural information is also utilized to understand and explain the molecular characteristics and interactions of modified, atherogenic LDL particles.

Electronegative LDL from normolipemic subjects induces IL-8 and monocyte chemotactic protein secretion by human endothelial cells

The presence in plasma of an electronegative LDL subfraction [LDL(-)] cytotoxic for endothelial cells (ECs) has been reported. We studied the effect of LDL(-) on the release by ECs of molecules implicated in leukocyte recruitment [interleukin-8 (IL-8) and monocyte chemotactic protein-1 (MCP-1)] and in the plasminogen activator inhibitor-1 (PAI-1). LDL(-), isolated by anion-exchange chromatography, differed from nonelectronegative LDL [LDL(+)] in its higher triglyceride, nonesterified fatty acid, apoprotein E and apoprotein C-III, and sialic acid contents. No evidence of extensive oxidation was found in LDL(-); its antioxidant and thiobarbituric acid-reactive substances contents were similar to those of LDL(+). However, conjugated dienes were increased in LDL(-), which suggests that mild oxidation might affect these particles. LDL(-) increased, in a concentration-dependent manner, the release of IL-8 and MCP-1 by ECs and was a stronger inductor of both chemokines than oxidized LDL (oxLDL) or LDL(+). PAI-1 release increased slightly in ECs incubated with both LDL(-) and oxLDL but not with LDL(+). However, no cytotoxic effects of LDL(-) were observed on ECs. Actinomycin D inhibited the release of IL-8 and MCP-1 induced by LDL(-) and oxLDL by up to 80%, indicating that their production is mediated by protein synthesis. Incubation of ECs with N:-acetyl cysteine inhibited production of IL-8 and MCP-1 induced by LDL(-) and oxLDL by >50%. The free radical scavenger butylated hydroxytoluene slightly inhibited the effect of oxLDL but did not modify the effect of LDL(-). An antagonist (BN-50730) of the platelet-activating factor receptor inhibited production of both chemokines by LDL(-) and oxLDL in a concentration-dependent manner. Our results indicate that LDL(-) shows proinflammatory activity on ECs and may contribute to early atherosclerotic events.

Postprandial lipemia and coronary risk

A number of cross-sectional studies have demonstrated that the magnitude of postprandial lipemia or single postprandial triglyceride values predict asymptomatic and symptomatic atherosclerosis, independent of risk factors measured in the fasting state. Postprandial lipemia reflects an integrated measure of an individual's triglyceride metabolic capacity. Numerous genetic and environmental factors that are known or suspected to affect triglyceride transport contribute to the magnitude of postprandial lipemia. In this article, mechanisms linking postprandial lipemia with the development and progression of atherosclerosis are described, and determinants of the extent and duration of postprandial lipemia are discussed.

Well-defined regions of apolipoprotein B-100 undergo conformational change during its intravascular metabolism

Apolipoprotein B (apoB)-100-containing lipoproteins are secreted from the liver as large triglyceride-rich very low density lipoproteins (VLDLs) into the circulation, where they are transformed, through the action of lipases and plasma lipid transfer proteins, into smaller, less buoyant, cholesteryl ester-rich low density lipoproteins (LDLs). As a consequence of this intravascular metabolism, apoB-containing lipoproteins are heterogeneous in size, in hydrated density, in surface charge, and in lipid and apolipoprotein composition. To identify specific regions of apoB that may undergo conformational changes during the intravascular transformation of VLDLs into LDLs, we have used a panel of 29 well-characterized anti-apoB monoclonal antibodies to determine whether individual apoB epitopes are differentially expressed in VLDL, intermediate density lipoprotein (IDL), and LDL subfractions isolated from 6 normolipidemic subjects. When analyzed in a solid-phase radioimmunoassay, the expression of most epitopes was remarkably similar in VLDLs, IDLs, and LDLs. Two epitopes that are close to the apoB LDL receptor-binding site show an increased expression in large (1.019 to 1.028 g/mL), medium (1.028 to 1.041 g/mL), and small (1.041 to 1.063 g/mL) LDLs compared with VLDLs and IDLs, and 2 epitopes situated between apoB residues 4342 and 4536 are significantly more immunoreactive in small and medium-sized LDLs compared with VLDLs, IDLs, and large LDLs. Therefore, as VLDL is converted to LDL, conformational changes identified by monoclonal antibodies occur at precise points in the metabolic cascade and are limited to well-defined regions of apoB structure. These conformational changes may correspond to alterations in apoB functional activities.

The post-prandial state and cardiovascular disease: relevance to diabetes mellitus

There is increasing evidence that the post-prandial state is an important contributing factor to the development of atherosclerosis. In non-diabetic subjects the atherosclerotic risk factors comprised in the categories of lipids, coagulation system and endothelial function may be adversely modified in the post-prandial phase. The generation of an oxidative stress may be the common pathway through which eating may induce these alterations. In diabetic patients these phenomena may be amplified by post-prandial hyperglycemia.

Low density lipoprotein (LDL) binding affinity for the LDL receptor in hyperlipoproteinemia

We measured the binding affinity of low density lipoprotein (LDL) for the LDL receptor in patients with various types of hyperlipoproteinemia and investigated the effects of LDL lipid composition and particle size on receptor affinity. LDL (1.019 < d < 1.063) was isolated by sequential ultracentrifugation from the serum of normolipidemic controls and patients with hyperlipoproteinemia. Patients with type IIa hyperlipoproteinemia had LDL with a similar receptor affinity to that of normal LDL. However, patients with hypertriglyceridemia (type IIb and type IV hyperlipoproteinemia) had LDL with a low receptor affinity, and the degree of the reduction in affinity paralleled the severity of the hypertriglyceridemia. The LDL of hypertriglyceridemic patients was rich in protein and triglycerides, had a low content of cholesterol and phospholipids, and was smaller than normal, thus resembling the atherogenic lipoprotein known as small, dense LDL. These abnormalities were observed even in patients with mild hypertriglyceridemia regardless of their serum cholesterol levels. The degree of alteration in LDL lipid composition and particle size was strongly associated with the reduction of LDL receptor affinity. We also examined the effects of two lipid-lowering agents (bezafibrate and probucol) on the characteristics of LDL. LDL receptor affinity was only improved when the lipid composition and particle size were normalized by drug therapy. Although it has been reported that decreased cholesteryl ester transfer protein (CETP) activity results in the formation of small LDL, plasma CETP activity was normal in the hyperlipoproteinemic patients and the normalization of LDL characteristics by drug therapy was not accompanied by an increase of CETP activity. Our results suggested that an abnormal lipid composition and/or small particle size might cause a decrease in the receptor affinity of LDL. These structural and functional abnormalities were reversed by drug therapy, underlining the importance of treating hypertriglyceridemia for the prevention of atherosclerosis.

Effects of native, triglyceride-enriched, and oxidatively modified LDL on plasminogen activator inhibitor-1 expression in human endothelial cells

Whereas VLDL has consistently been shown to induce a concentration-dependent increase in the expression of plasminogen activator inhibitor-1 (PAI-1) in human umbilical vein endothelial cells (HUVECs) and liver cells, variable effects have been reported for native and oxidatively modified LDL. In the present study, activation of PAI-1 protein and mRNA expression by native LDL (nLDL), UV-oxidized LDL (uvLDL), and triglyceride (TG)-enriched LDL was studied in HUVECs by using different incubation times and a wide range of lipoprotein concentrations. No significant increase of PAI-1 protein expression was observed after 4 hours of incubation with nLDL or uvLDL. However, PAI-1 protein secretion from HUVECs was markedly enhanced after 18 hours of incubation with uvLDL (200% increase at 10 microg/mL). Stimulation of PAI-1 protein expression in HUVECs by nLDL was seen, however, after increasing the TG content of the LDL particle. LDL enriched in phospholipid had no effect on PAI-1 secretion. PAI-1 mRNA levels on northern blot increased in parallel with the activation of PAI-1 protein expression by native and modified forms of LDL. Low concentrations of TG-enriched LDL (10 microg/mL) and higher concentrations of nLDL and uvLDL (100 microg/mL) were found to increase the binding of a VLDL-inducible transcription factor to the PAI-1 promoter. These results indicate that the TG content of the LDL particle influences PAI-1 expression in endothelial cells. Low concentrations of uvLDL enhanced PAI-1 protein and mRNA expression in the HUVECs after an 18-hour incubation but did not influence the VLDL-inducible transcription factor. This suggests that low levels of oxidized LDL increase PAI-1 expression by a different mechanism than VLDL and TG-enriched LDL.

Correlation of serum triglyceride and its reduction by omega-3 fatty acids with lipid transfer activity and the neutral lipid compositions of high-density and low-density lipoproteins

Serum triglyceride (TG) and high-density lipoprotein cholesterol (HDL-C) concentrations are inversely correlated and mechanistically linked by means of lipid transfer activities. Phospholipid transfer activity (PLTA) moves phospholipids among serum lipoproteins; cholesteryl ester transfer activity (CETA), which exchanges cholesteryl esters (CE) and TG among lipoproteins, is stimulated by nonesterified fatty acids (NEFA). The aims of this study were (a) to develop a quantitative model that correlates the neutral lipid (NL = CE + TG) compositions of HDL and LDL with serum TG concentration; (b) identify the serum lipid determinants of CETA and PLTA, and; (c) identify the effects of serum TG reductions on the neutral lipid compositions of HDL and LDL, serum NEFA concentrations, and on PLTA and CETA. These aims were addressed in 40 hypertriglyceridemic subjects before and after treatment with an 85% concentrate of omega-3 fatty acids (Omacor) and in 16 untreated normolipidemic subjects. In vivo, the NL compositions of LDL and HDL were described by a mathematical model having the form of adsorption isotherms: HDL - (TG/NL) = (0.90 +/- 0.07) serum TG/(7.0 +/- 1.2 mmol/l + serum TG) and LDL - (TG/NL) = (0.65 +/- 0.08) serum TG/(4.9 +/- 1.5 mmol/l + serum TG). Reduction of serum TG was associated with reductions in HDL - (TG/NL), serum NEFA concentration, and serum CETA but not PLTA. These data suggest that both hypertriglyceridemia and the attendant elevated serum CETA but not PLTA are determinants of HDL and LDL composition and structure and that serum TG concentrations are good predictors of the NL compositions of HDL and LDL.

Comparison of the LDL-receptor binding of VLDL and LDL from apoE4 and apoE3 homozygotes

Compared with apolipoprotein E3 (apoE3), apoE2 is less effective in mediating the binding of lipoproteins to the low-density lipoprotein (LDL) receptor. The influence of the E4 isoform, which is associated with adverse effects on plasma lipids and coronary heart disease, is less clear. We compared the ability of very low density lipoprotein (VLDL) and LDL from paired E4/4 and E3/3 subjects to compete against 125I-labeled LDL for binding with the LDL receptor on cultured fibroblasts and Hep G2 cells. The concentrations of VLDL or LDL required to inhibit binding of 125I-LDL by 50% (IC50, microgram apoB/ml) were determined, and results were assessed in terms of an IC50 ratio, E4/4 IC50 relative to E3/3 IC50, to reduce the influence of interassay variability. In Hep G2 cells, E4/4 VLDL was more effective than E3/3 VLDL in competing for the LDL receptor, the IC50 ratio being lower than unity (0.73 +/- 0.31, P < 0.05, two-tailed t-test). IC50 values themselves were marginally lower in E4/4 than E3/3 subjects (3.7 +/- 1.3 vs. 6.1 +/- 3.7, P < 0.08). However, there was no difference between E4/4 and E3/3 VLDL in competing for the LDL receptor on fibroblasts or between E4/4 and E3/3 LDL in competing for the LDL receptor on either cell type. These results suggest that inheritance of apoE4 is associated with an increased affinity of VLDL particles for LDL receptors on hepatocytes and may partly explain the influence of the E4 isoform on lipid metabolism.

Prediction of preeclampsia

The onset of preeclampsia at or near to term is associated with low maternal and neonatal morbidity and mortality. In contrast, those patients (1%) who suffer early onset preeclampsia engender significant maternal and perinatal morbidity and mortality. Therefore, because of the lack of proven prophylaxis for preeclampsia, prediction of risk or identification of subclinical disease is desirable to identify patients for more intensive observation. There are certain at-risk groups of patients such as those with chronic hypertension, pregestational diabetes, multifetal gestation, and previous preeclampsia. These patients account for the majority of cases of preeclampsia in multiparas, yet only account for 14% of preeclampsia in nulliparous women. Thus, the majority of cases of preeclampsia arises from nulliparous women without medical complications at low risk. Differences in the time of onset, severity, and organ system involvement suggest there may be different underlying etiologies that ultimately lead to preeclampsia manifested as the triad of maternal hypertension, proteinuria, and edema. Distinct markers therefore may identify subgroups of at-risk patients with separate underlying causes. These markers ultimately could be used for diagnosis of disease before the clinical appearance of maternal disease (hypertension, proteinuria, and edema). Based on data from patients with established disease, with the involvement of various organ systems, potential candidate markers would include renal function (kallikrein-creatinine); coagulation and fibrinolytic systems and platelet activation (platelet volume); markers of vascular function (fibronectin, prostacyclin, thromboxane) and oxidant stress (lipid peroxides, 8-isoprostane, antioxidants, anticardiolipin antibodies, hemoglobin, iron, transferrin, homocysteine, hypertriglyceridemia, albumin isoforms); placental peptide hormones (CRH, CRHbp, activin, inhibin, hCG); vascular resistance (uteroplacental flow velocity waveforms); genetic markers; insulin resistance; and glucose intolerance. Although cross-sectional studies have identified these potential markers, they need to be evaluated in prospective longitudinal studies with rigorous definition of outcome to determine if they are useful in predicting preeclampsia and whether they can identify different subgroups of patients.

In vitro production of beta-very low density lipoproteins and small, dense low density lipoproteins in mildly hypertriglyceridemic plasma: role of activities of lecithin:cholester acyltransferase, cholesterylester transfer proteins and lipoprotein lipase

As a model for the formation of beta-very low density lipoproteins (VLDL) and small, dense LDL by the intraplasma metabolic activities in vivo, lipoproteins in fresh plasma were interacted in vitro with endogenous lecithin:cholesterol acyltransferase (LCAT) and cholesterylester transfer proteins (CETP) and subsequently with purified lipoprotein lipase (LpL). The LCAT and CETP reactions in a mildly hypertriglyceridemic (HTG) plasma at 37 degrees C for 18 h resulted in (1) esterification of about 45% plasma unesterified cholesterol (UC), (2) a marked increase in cholesterylester (CE) (+129%) and a decrease in triglyceride (TG) (-45%) in VLDL, and (3) a marked increase of TG (+ 341%) with a small net decrease of CE (-3.6%) in LDL, causing a significant alteration in the TG/CE of VLDL (from 8.0 to 1.9) and of LDL (from 0.20 to 0.93). The LDL in LCAT and CETP-reacted plasma is larger and more buoyant than that in control plasma. In vitro lipolysis of control and LCAT and CETP-reacted plasma by LpL, which hydrolyzed >90% of VLDL-TG and about 50-60% of LDL-TG, converted most of VLDL in control plasma (>85%) but less than half (40%) of VLDL in LCAT and CETP-reacted plasma into the IDL-LDL density fraction and transformed the large, buoyant LDL in the LCAT and CETP-reacted plasma into particles smaller and denser than those in the control plasma. The remnants that accumulated in the VLDL density region of the postlipolysis LCAT and CETP-reacted plasma contained apo B-100 and E but little or no detectable apo Cs and consisted of particles having pre-beta and beta-electrophoretic mobilities. The inhibition of LCAT during incubation of plasma, which lessened the extent of alteration in VLDL and LDL core lipids, increased the extent of lipolytic removal of VLDL from the VLDL density region but lowered the extent of alteration in the size and density of LDL. The LCAT, CETP and/or LpL-mediated alterations in the density of LDL in normolipidemic fasting plasma were less pronounced than that in mildly HTG plasma, but they became highly pronounced upon increase of its TG-rich lipoprotein level by the addition of preisolated VLDL or by the induction of postprandial lipemia. Although the effect of LCAT, CETP and LpL reactions in non-circulating plasma in vitro may be different from that in vivo, the above data suggests that the plasma TG-rich lipoprotein level and the extent of intraplasma LCAT, CETP, LpL and likely hepatic lipase (HL) reactions in vivo may play a role in determining the LDL phenotype.

Elevated LDL triglyceride concentrations in subjects heterozygous for the hepatic lipase S267F variant

Although naturally occurring loss-of-function mutations in human hepatic lipase (HL) have been described, the biochemical phenotype of heterozygous HL deficiency remains ill defined. This may be due to the relatively small numbers of heterozygous adult carriers of HL mutations in index kindreds. We have identified several new heterozygotes for the catalytically inactive, nonsecreted HL variant S267F in the kindred that was originally ascertained because of hypertriglyceridemia due to the mutant, secreted, circulating apolipoprotein (apo) CII variant apo CII-T. Pairwise comparisons with family controls showed that only the plasma low density lipoprotein triglycerides (LDL TGs) were higher in 11 simple heterozygotes for HL S267F (P=0.002). In contrast, both plasma total TGs and LDL TGs were significantly higher in 12 simple heterozygotes for apo CII-T than in family-matched control subjects (P=0.005 and 0.009, respectively). These findings suggest that the TG content of LDL is increased by heterozygosity for 2 different mutations that affect different proteins involved in lipolysis. However, the mechanisms underlying this compositional change in LDL appear to be different for the 2 mutations, because the total TGs are also elevated in subjects heterozygous for apo CII-T but not in subjects heterozygous for HL S267F.

Increased expression of apolipoprotein E in transgenic rabbits results in reduced levels of very low density lipoproteins and an accumulation of low density lipoproteins in plasma

Transgenic rabbits expressing human apo E3 were generated to investigate mechanisms by which apo E modulates plasma lipoprotein metabolism. Compared with nontransgenic littermates expressing approximately 3 mg/dl of endogenous rabbit apo E, male transgenic rabbits expressing approximately 13 mg/dl of human apo E had a 35% decrease in total plasma triglycerides that was due to a reduction in VLDL levels and an absence of large VLDL. With its greater content of apo E, transgenic VLDL had an increased binding affinity for the LDL receptor in vitro, and injected chylomicrons were cleared more rapidly by the liver in transgenic rabbits. In contrast to triglyceride changes, transgenic rabbits had a 70% increase in plasma cholesterol levels due to an accumulation of LDL and apo E-rich HDL. Transgenic and control LDL had the same binding affinity for the LDL receptor. Both transgenic and control rabbits had similar LDL receptor levels, but intravenously injected human LDL were cleared more slowly in transgenic rabbits than in controls. Changes in lipoprotein lipolysis did not contribute to the accumulation of LDL or the reduction in VLDL levels. These observations suggest that the increased content of apo E3 on triglyceride-rich remnant lipoproteins in transgenic rabbits confers a greater affinity for cell surface receptors, thereby increasing remnant clearance from plasma. The apo E-rich large remnants appear to compete more effectively than LDL for receptor-mediated binding and clearance, resulting in delayed clearance and the accumulation of LDL in plasma.

Lipoprotein heterogeneity and apolipoprotein B metabolism

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