Lipoprotein lipase in human plasma is mainly inactive and associated with cholesterol-rich lipoproteins

Macromolecular Interactions of Lipoprotein Lipase (LPL)

Lipoprotein lipase (LPL) is a critical enzyme in humans that provides fuel to peripheral tissues. LPL hydrolyzes triglycerides from the cores of lipoproteins that are circulating in plasma and interacts with receptors to mediate lipoprotein uptake, thus directing lipid distribution via catalytic and non-catalytic functions. Functional losses in LPL or any of its myriad of regulators alter lipid homeostasis and potentially affect the risk of developing cardiovascular disease-either increasing or decreasing the risk depending on the mutated protein. The extensive LPL regulatory network tunes LPL activity to allocate fatty acids according to the energetic needs of the organism and thus is nutritionally responsive and tissue dependent. Multiple pharmaceuticals in development manipulate or mimic these regulators, demonstrating their translational importance. Another facet of LPL biology is that the oligomeric state of the enzyme is also central to its regulation. Recent structural studies have solidified the idea that LPL is regulated not only by interactions with other binding partners but also by self-associations. Here, we review the complexities of the protein-protein and protein-lipid interactions that govern LPL structure and function.

Structure of dimeric lipoprotein lipase reveals a pore adjacent to the active site

Lipoprotein lipase (LPL) hydrolyzes triglycerides from circulating lipoproteins, releasing free fatty acids. Active LPL is needed to prevent hypertriglyceridemia, which is a risk factor for cardiovascular disease (CVD). Using cryogenic electron microscopy (cryoEM), we determined the structure of an active LPL dimer at 3.9 Å resolution. This structure reveals an open hydrophobic pore adjacent to the active site residues. Using modeling, we demonstrate that this pore can accommodate an acyl chain from a triglyceride. Known LPL mutations that lead to hypertriglyceridemia localize to the end of the pore and cause defective substrate hydrolysis. The pore may provide additional substrate specificity and/or allow unidirectional acyl chain release from LPL. This structure also revises previous models on how LPL dimerizes, revealing a C-terminal to C-terminal interface. We hypothesize that this active C-terminal to C-terminal conformation is adopted by LPL when associated with lipoproteins in capillaries.

Structure of Dimeric Lipoprotein Lipase Reveals a Pore for Hydrolysis of Acyl Chains

Lipoprotein lipase (LPL) hydrolyzes triglycerides from circulating lipoproteins, releasing free fatty acids. Active LPL is needed to prevent hypertriglyceridemia, which is a risk factor for cardiovascular disease (CVD). Using cryogenic electron microscopy (cryoEM), we determined the structure of an active LPL dimer at 3.9 Ã… resolution. This is the first structure of a mammalian lipase with an open, hydrophobic pore adjacent to the active site. We demonstrate that the pore can accommodate an acyl chain from a triglyceride. Previously, it was thought that an open lipase conformation was defined by a displaced lid peptide, exposing the hydrophobic pocket surrounding the active site. With these previous models after the lid opened, the substrate would enter the active site, be hydrolyzed and then released in a bidirectional manner. It was assumed that the hydrophobic pocket provided the only ligand selectivity. Based on our structure, we propose a new model for lipid hydrolysis, in which the free fatty acid product travels unidirectionally through the active site pore, entering and exiting opposite sides of the protein. By this new model, the hydrophobic pore provides additional substrate specificity and provides insight into how LPL mutations in the active site pore may negatively impact LPL activity, leading to chylomicronemia. Structural similarity of LPL to other human lipases suggests that this unidirectional mechanism could be conserved but has not been observed due to the difficulty of studying lipase structure in the presence of an activating substrate. We hypothesize that the air/water interface formed during creation of samples for cryoEM triggered interfacial activation, allowing us to capture, for the first time, a fully open state of a mammalian lipase. Our new structure also revises previous models on how LPL dimerizes, revealing an unexpected C-terminal to C-terminal interface. The elucidation of a dimeric LPL structure highlights the oligomeric diversity of LPL, as now LPL homodimer, heterodimer, and helical filament structures have been elucidated. This diversity of oligomerization may provide a form of regulation as LPL travels from secretory vesicles in the cell, to the capillary, and eventually to the liver for lipoprotein remnant uptake. We hypothesize that LPL dimerizes in this active C-terminal to C-terminal conformation when associated with mobile lipoproteins in the capillary.

Effect of Siberian Ginseng Water Extract as a Dietary Additive on Growth Performance, Blood Biochemical Indexes, Lipid Metabolism, and Expression of PPARs Pathway-Related Genes in Genetically Improved Farmed Tilapia (Oreochromis niloticus)

Overnutrition in high-density aquaculture can negatively affect the health of farmed fish. The Chinese herbal medicine Siberian ginseng (Acanthopanax senticosus, AS) can promote animal growth and immunity, and regulate lipid metabolism. Therefore, we conducted an 8-week experiment, in which Oreochromis niloticus was fed with a diet supplemented with different concentrations of AS water extract (ASW) (0‰, 0.1‰, 0.2‰, 0.4‰, 0.8‰, and 1.6‰). The ASW improved the growth performance and increased the specific growth rate (SGR). Linear regression analysis based on the SGR estimated that the optimal ASW amount was 0.74‰. Dietary supplementation with 0.4–0.8‰ ASW reduced the triglyceride and total cholesterol levels in the serum and liver, and regulated lipid transport by increasing the high-density lipoprotein cholesterol concentration and lowering the low-density lipoprotein cholesterol concentration. Dietary supplementation with ASW increased the activities of superoxide dismutase and catalase in the liver, thereby improving the antioxidant capacity. Moreover, ASW modulated the transcription of genes in the peroxisome proliferator-activated receptor signaling pathway in the liver (upregulation of PPARα, APOA1b, and FABP10a and downregulation of PPARγ), thereby regulating fatty acid synthesis and metabolism and slowing fat deposition. These results showed that 0.4–0.8‰ ASW can slow fat deposition and protected the liver from cell damage and abnormal lipid metabolism.

Lipoprotein size is a main determinant for the rate of hydrolysis by exogenous LPL in human plasma

LPL is a key player in plasma triglyceride metabolism. Consequently, LPL is regulated by several proteins during synthesis, folding, secretion, and transport to its site of action at the luminal side of capillaries, as well as during the catalytic reaction. Some proteins are well known, whereas others have been identified but are still not fully understood. We set out to study the effects of the natural variations in the plasma levels of all known LPL regulators on the activity of purified LPL added to samples of fasted plasma taken from 117 individuals. The enzymatic activity was measured at 25°C using isothermal titration calorimetry. This method allows quantification of the ability of an added fixed amount of exogenous LPL to hydrolyze triglyceride-rich lipoproteins in plasma samples by measuring the heat produced. Our results indicate that, under the conditions used, the normal variation in the endogenous levels of apolipoprotein C1, C2, and C3 or the levels of angiopoietin-like proteins 3, 4, and 8 in the fasted plasma samples had no significant effect on the recorded activity of the added LPL. Instead, the key determinant for the LPL activity was a lipid signature strongly correlated to the average size of the VLDL particles. The signature involved not only several lipoprotein and plasma lipid parameters but also apolipoprotein A5 levels. While the measurements cannot fully represent the action of LPL when attached to the capillary wall, our study provides knowledge on the interindividual variation of LPL lipolysis rates in human plasma.

The Importance of Lipoprotein Lipase Regulation in Atherosclerosis

Lipoprotein lipase (LPL) plays a major role in the lipid homeostasis mainly by mediating the intravascular lipolysis of triglyceride rich lipoproteins. Impaired LPL activity leads to the accumulation of chylomicrons and very low-density lipoproteins (VLDL) in plasma, resulting in hypertriglyceridemia. While low-density lipoprotein cholesterol (LDL-C) is recognized as a primary risk factor for atherosclerosis, hypertriglyceridemia has been shown to be an independent risk factor for cardiovascular disease (CVD) and a residual risk factor in atherosclerosis development. In this review, we focus on the lipolysis machinery and discuss the potential role of triglycerides, remnant particles, and lipolysis mediators in the onset and progression of atherosclerotic cardiovascular disease (ASCVD). This review details a number of important factors involved in the maturation and transportation of LPL to the capillaries, where the triglycerides are hydrolyzed, generating remnant lipoproteins. Moreover, LPL and other factors involved in intravascular lipolysis are also reported to impact the clearance of remnant lipoproteins from plasma and promote lipoprotein retention in capillaries. Apolipoproteins (Apo) and angiopoietin-like proteins (ANGPTLs) play a crucial role in regulating LPL activity and recent insights into LPL regulation may elucidate new pharmacological means to address the challenge of hypertriglyceridemia in atherosclerosis development.

Genetic Ablation of Transmembrane Prolyl 4-Hydroxylase Reduces Atherosclerotic Plaques in Mice

[Figure: see text].

Maternal Adipose Tissue Expansion, A Missing Link in the Prediction of Birth Weight Centile

CONTEXT

Maternal body mass index (BMI) is associated with increased birth weight but does not explain all the variance in fetal adiposity.

OBJECTIVE

To assess the contribution of maternal body fat distribution to offspring birth weight and adiposity.

DESIGN

Longitudinal study throughout gestation and at delivery.

SETTING

Women recruited at 12 weeks of gestation and followed up at 26 and 36 weeks. Cord blood was collected at delivery.

PATIENTS

Pregnant women (n = 45) with BMI 18.0 to 46.3 kg/m2 and healthy pregnancy outcome.

METHODS

Maternal first trimester abdominal subcutaneous and visceral adipose tissue thickness (SAT and VAT) was assessed by ultrasound.

MAIN OUTCOME MEASURES

Maternal body fat distribution, maternal and cord plasma glucose and lipid concentrations, placental weight, birth weight, and fetal adiposity assessed by cord blood leptin.

RESULTS

VAT was the only anthropometric measure independently associated with birth weight centile (r2 adjusted 15.8%, P = .002). BMI was associated with trimester 2 and trimesters 1 through 3 area under the curve (AUC) glucose and insulin resistance (Homeostatic Model Assessment). SAT alone predicted trimester 2 lipoprotein lipase (LPL) mass (a marker of adipocyte insulin sensitivity) (11.3%, P = .017). VAT was associated with fetal triglyceride (9.3%, P = .047). Placental weight was the only independent predictor of fetal adiposity (48%, P < .001). Maternal trimester 2 and AUC LPL were inversely associated with fetal adiposity (r = -0.69, P = .001 and r = -0.58, P = .006, respectively).

CONCLUSIONS

Maternal VAT provides additional information to BMI for prediction of birth weight. VAT may be a marker of reduced SAT expansion and increased availability of maternal fatty acids for placental transport.

Apolipoprotein C-II mimetic peptide is an efficient activator of lipoprotein lipase in human plasma as studied by a calorimetric approach

Elevated plasma triglyceride (TG) levels are associated with higher risk of atherosclerotic cardiovascular disease. One way to reduce plasma TG is to increase the activity of lipoprotein lipase (LPL), the rate limiting enzyme in plasma TG metabolism. An apolipoprotein (apo) C-II mimetic peptide (18A-CII-a) has been recently developed that stimulated LPL activity in vitro and decreased plasma TG concentration in animal models for hypertriglyceridemia. Since this peptide can serve as a new therapeutic approach for treatment of hypertriglyceridemia, we investigated how 18A-CII-a peptide influences LPL activity in human plasma. We used recently described isothermal titration calorimetry based approach to assess the peptide, which enables the analysis in nearly undiluted human plasma. The 18A-CII-a peptide was 3.5-fold more efficient in stimulating LPL activity than full-length apoC-II in plasma sample from normolipidemic individual. Furthermore, 18A-CII-a also increased LPL activity in hypertriglyceridemic plasma samples. Unlike apoC-II, high concentrations of the 18A-CII-a peptide did not inhibit LPL activity. The increase in LPL activity after addition of 18A-CII-a or apoC-II to plasma was due to the increase of the amount of available substrate for LPL. Measurements with isolated lipoproteins revealed that the relative activation effects of 18A-CII-a and apoC-II on LPL activity were greater in smaller size lipoprotein fractions, such as remnant lipoproteins, low-density lipoproteins and high-density lipoproteins. In summary, this report describes a novel mechanism of action for stimulation of LPL activity by apoC-II mimetic peptides.

High Concentrations of Angiopoietin-Like Protein 4 Detected in Serum from Patients with Rheumatoid Arthritis Can Be Explained by Non-Specific Antibody Reactivity

Angiopoietin-like protein 4 (ANGPTL4) is suggested to be a master regulator of plasma triglyceride metabolism. Our aim was to study whether the previously reported high levels of ANGPTL4 detected in serum from patients with rheumatoid arthritis (RA) by ELISA was due to any specific molecular form of this protein (oligomers, monomers or fragments). ANGPTL4 levels were first determined in serum from 68 RA patients and 43 age and sex matched control subjects and the mean values differed by a factor of 5.0. Then, ANGPTL4 was analyzed after size exclusion chromatography (SEC) of serum samples. With serum from one of the RA patients with high levels of ANGPTL4, the dominant reactivity was found in fractions corresponding to high-molecular weight proteins. In addition, a minor peak of reactivity eluting late from the column was found both in the patient and in controls. By the use of HeteroBlock®, and by careful selection of antibodies, we documented non-specific reactions for ANGPTL4 in 39% of samples from the RA patients, most likely due to cross-reactivity of the antibodies with rheumatoid factor (RF). The corresponding figure for control subjects was 6.3%. After corrections for non-specific reactions, the mean level of ANGPTL4 in serum from RA patients was still significantly higher than in control individuals (mean levels were 101±62 and 67±39 ng/ml respectively, P = 0.02). We re-analyzed samples from our previously published studies on ANGPL4 levels in patients on hemodialysis and patients with diabetes type 2. These samples did not show false positive reactions. The levels of ANGPTL4 were comparable to those detected previously.

Kinetic and Related Determinants of Plasma Triglyceride Concentration in Abdominal Obesity: Multicenter Tracer Kinetic Study

OBJECTIVES

Patients with obesity and diabetes mellitus have increased risk of cardiovascular disease. A major cause is an atherogenic dyslipidemia related primarily to elevated plasma concentrations of triglyceride-rich lipoproteins. The aim of this study was to clarify determinants of plasma triglyceride concentration. We focused on factors that predict the kinetics of very-low density lipoprotein 1 (VLDL1) triglycerides.

APPROACH AND RESULTS

A multicenter study using dual stable isotopes (deuterated leucine and glycerol) and multicompartmental modeling was performed to elucidate the kinetics of triglycerides and apoB in VLDL1 in 46 subjects with abdominal obesity and additional cardiometabolic risk factors. Results showed that plasma triglyceride concentrations were dependent on both the secretion rate (r=0.44, P<0.01; r=0.45, P<0.01) and fractional catabolism (r=0.49, P<0.001; r=0.55, P<0.001) of VLDL1-triglycerides and VLDL1-apoB. Liver fat mass was independently and directly associated with secretion rates of VLDL1-triglycerides (r=0.56, P<0.001) and VLDL1-apoB (r=0.53, P<0.001). Plasma apoC-III concentration was independently and inversely associated with the fractional catabolisms of VLDL1-triglycerides (r=0.48, P<0.001) and VLDL1-apoB (r=0.51, P<0.001).

CONCLUSIONS

Plasma triglyceride concentrations in abdominal obesity are determined by the kinetics of VLDL1 subspecies, catabolism being mainly dependent on apoC-III concentration and secretion on liver fat content. Reduction in liver fat and targeting apoC-III may be an effective approach for correcting triglyceride metabolism atherogenic dyslipidemia in obesity.

Current knowledge of hypertriglyceridemic pancreatitis

Severe hypertriglyceridemia (HTG) is a well established and the most common cause of acute pancreatitis (AP) after alcohol and gall stone disease. It is alleged to account for up to 10% of all pancreatitis episodes. Studies suggest that in patients with triglyceride (TG) levels>1000 mg/dL (>11.3 mmol/L), hypertriglyceridemia-induced acute pancreatitis (HTGP-AP) occurs in approximately 15-20% of all subjects referred to Lipid Clinics. Until now, there is no clear evidence which patients with severe HTG will develop pancreatitis and which will not. Underlying pathophysiological concepts include hydrolysis of TG by pancreatic lipase and excessive formation of free fatty acids with inflammatory changes and capillary injury. Additionally hyperviscosity and ischemia may play a decisive role. The clinical features of HTG-AP patients are supposed to be no different from patients with AP of other etiologies. Yet, there are well-conducted studies suggesting that HTG-AP is associated with a higher severity and complication rate. Therapeutic measurements in HTG-AP include dietary modifications, different antihyperlipidemic agents, insulin and/or heparin treatment. The beneficial use of plasmapheresis is repeatedly reported and suggested in many studies. Yet, due to the lack of randomized and controlled trials, it is currently unknown if plasmapheresis may improve morbidity and mortality in the clinical setting of HTG-AP. Since there are no commonly accepted clinical guidelines in the management of HTG-AP, there is a definite need for an international, multicenter approach to this important subject.

Fasting apolipoprotein B48 is a marker for peripheral arterial disease in type 2 diabetes

An earlier study showed that fasting and postprandial concentrations of apolipoprotein B48 were raised in patients with type 2 diabetes (DM2) and peripheral arterial disease (PAD) as compared with persons without DM2 or persons with DM2 but not PAD. The aim of this study was to confirm the association of PAD and B48 in a larger group of patients with DM2 and the relation of B48 with the preheparin lipoprotein lipase (LPL) mass. We studied 456 patients with DM2. PAD was defined as an ankle-brachial index (ABI) <0.9. Apolipoprotein B48 was quantified by ELISA. Apo B48 was significantly higher in the group with an ABI <0.9 than the groups with ABI of 0.9-1.3 and >1.3 (10.7 ± 6.28 vs. 9.24 ± 5.5 vs. 9.17 ± 8.8 mg/L, ANOVA test, p < 0.05). B48 was independently associated with an ABI <0.9 (OR 1.053; 95 % CI, 1.013-1.094; p < 0.05), together with smoking and duration of diabetes. The preheparin LPL mass was similar in the patients with and without PAD. In conclusion, we confirmed that fasting B48 is an independent marker of PAD in patients with DM2, unrelated to the preheparin LPL mass, statin therapy or glucose lowering treatment.

Secretion of adipokines by human adipose tissue in vivo: partitioning between capillary and lymphatic transport

Peptides secreted by adipose tissue (adipokines) may enter blood via capillaries or lymph. The relative importance of these pathways for a given adipokine might influence its biological effects. Because this has not been studied in any species, we measured the concentrations of seven adipokines and eight nonsecreted proteins in afferent peripheral lymph and venous plasma from 12 healthy men. Data for nonsecreted proteins were used to derive indices of microvascular permeability, which in conjunction with the molecular radii of the adipokines were used to estimate the amounts leaving the tissue via capillaries. Transport rates via lymph were estimated from the lymph adipokine concentrations and lymph flow rates and total transport (secretion) as the sum of this and capillary transport. Concentrations of nonsecreted proteins were always lower in lymph than in plasma. With the exception of adiponectin, adipokine concentrations were always higher in lymph (P < 0.01). Leptin and MCP-1 were secreted at the highest rates (means: 43 μg/h or 2.7 nmol/h and 32 μg/h or 2.4 nmol/h, respectively). IL-6 and MCP-1 secretion rates varied greatly between subjects. The proportion of an adipokine transported via lymph was directly related to its molecular radius (r(s) = +0.94, P = 0.025, n = 6), increasing from 14 to 100% as the radius increased from 1.18 (IL-8) to 3.24 nm (TNFα). We conclude that the lymph/capillary partitioning of adipokines is a function of molecular size, which may affect both their regional and systemic effects in vivo. This finding may have implications for the physiology of peptides secreted by other tissues.

Mutation of conserved cysteines in the Ly6 domain of GPIHBP1 in familial chylomicronemia

We investigated a family from northern Sweden in which three of four siblings have congenital chylomicronemia. LPL activity and mass in pre- and postheparin plasma were low, and LPL release into plasma after heparin injection was delayed. LPL activity and mass in adipose tissue biopsies appeared normal. [(35)S]Methionine incorporation studies on adipose tissue showed that newly synthesized LPL was normal in size and normally glycosylated. Breast milk from the affected female subjects contained normal to elevated LPL mass and activity levels. The milk had a lower than normal milk lipid content, and the fatty acid composition was compatible with the milk lipids being derived from de novo lipogenesis, rather than from the plasma lipoproteins. Given the delayed release of LPL into the plasma after heparin, we suspected that the chylomicronemia might be caused by mutations in GPIHBP1. Indeed, all three affected siblings were compound heterozygotes for missense mutations involving highly conserved cysteines in the Ly6 domain of GPIHBP1 (C65S and C68G). The mutant GPIHBP1 proteins reached the surface of transfected Chinese hamster ovary cells but were defective in their ability to bind LPL (as judged by both cell-based and cell-free LPL binding assays). Thus, the conserved cysteines in the Ly6 domain are crucial for GPIHBP1 function.

Lipoprotein lipase is differentially expressed in prognostic subsets of chronic lymphocytic leukemia but displays invariably low catalytical activity

Lipoprotein lipase (LPL) expression has been shown to correlate with IGHV mutational status and to predict outcome in chronic lymphocytic leukemia (CLL). We here investigated the prognostic impact of LPL expression in relation to other prognostic markers including IGHV3-21 usage in 140 CLL patients. Additionally, we studied the catalytic activity of LPL in CLL cells. A significant difference in LPL mRNA expression was detected in IGHV unmutated compared to mutated CLL patients (p<0.001). However, the poor-prognostic mutated/stereotyped IGHV3-21 patients did not differ from other mutated CLL cases. Clinical outcome was significantly different in CLL cases with high versus low LPL expression (p<0.001), and LPL expression exceeded mutation status/IGHV3-21 usage as an independent prognostic marker. Finally, LPL protein expression correlated significantly with mRNA expression and was higher in IGHV unmutated versus mutated CLL (p=0.018), although the majority of synthesized protein was catalytically inactive indicating a non-catalytical function in CLL.

Deficiency in monocyte chemoattractant protein-1 modifies lipid and glucose metabolism

We describe the effect of MCP-1 deficiency in mice rendered hyperlipemic by the concomitant ablation of the LDL receptor. The MCP-1(-/-)LDLr(-/-) mice in comparison with LDLr(-/-) mice showed a decreased lipoprotein clearance, derangements in free fatty acids delivery and less glucose tolerance when fed a regular chow, and they showed a partial resistance to alterations in glucose and lipid metabolism induced by dietary fat and cholesterol. They also were less prone to the development of diet-induced obesity. Our results suggest that the role of MCP-1 in metabolism is relevant and that, although new hidden complexities are evident, the function of MCP-1/CCL2 extends far beyond the monocyte chemoattractant effect. Therefore, the regulatory mechanisms influenced by MCP-1 should be fully ascertained to understand the metabolic consequences of inflammation and before considering MCP-1 as a therapeutic target.

Angptl4 upregulates cholesterol synthesis in liver via inhibition of LPL- and HL-dependent hepatic cholesterol uptake

BACKGROUND

Dysregulation of plasma lipoprotein levels may increase the risk for atherosclerosis. Recently, angiopoietin-like protein 4, also known as fasting-induced adipose factor Fiaf, was uncovered as a novel modulator of plasma lipoprotein metabolism. Here we take advantage of the fasting-dependent phenotype of Angptl4-transgenic (Angptl4-Tg) mice to better characterize the metabolic function of Angptl4.

METHODS AND RESULTS

In 24-hour fasted mice, Angptl4 overexpression increased plasma triglycerides (TG) by 24-fold, which was attributable to elevated VLDL-, IDL/LDL- and HDL-TG content. Angptl4 overexpression decreased post-heparin LPL activity by stimulating conversion of endothelial-bound LPL dimers to circulating LPL monomers. In fasted but not fed state, Angptl4 overexpression severely impaired LPL-dependent plasma TG and cholesteryl ester clearance and subsequent uptake of fatty acids and cholesterol into tissues. Consequently, hepatic cholesterol content was significantly decreased, leading to universal upregulation of cholesterol and fatty acid synthesis pathways and increased rate of cholesterol synthesis.

CONCLUSIONS

The hypertriglyceridemic effect of Angptl4 is attributable to inhibition of LPL-dependent VLDL lipolysis by converting LPL dimers to monomers, and Angptl4 upregulates cholesterol synthesis in liver secondary to inhibition of LPL- and HL-dependent hepatic cholesterol uptake.

Identification and characterization of peptides that interact with hepatitis B virus via the putative receptor binding site

A direct involvement of the PreS domain of the hepatitis B virus (HBV) large envelope protein, and in particular amino acid residues 21 to 47, in virus attachment to hepatocytes has been suggested by many previous studies. Several PreS-interacting proteins have been identified. However, they share few common sequence motifs, and a bona fide cellular receptor for HBV remains elusive. In this study, we aimed to identify PreS-interacting motifs and to search for novel HBV-interacting proteins and the long-sought receptor. PreS fusion proteins were used as baits to screen a phage display library of random peptides. A group of PreS-binding peptides were obtained. These peptides could bind to amino acids 21 to 47 of PreS1 and shared a linear motif (W1T2X3W4W5) sufficient for binding specifically to PreS and viral particles. Several human proteins with such a motif were identified through BLAST search. Analysis of their biochemical and structural properties suggested that lipoprotein lipase (LPL), a key enzyme in lipoprotein metabolism, might interact with PreS and HBV particles. The interaction of HBV with LPL was demonstrated by in vitro binding, virus capture, and cell attachment assays. These findings suggest that LPL may play a role in the initiation of HBV infection. Identification of peptides and protein ligands corresponding to LPL that bind to the HBV envelope will offer new therapeutic strategies against HBV infection.

Serum lipoprotein lipase mass: clinical significance of its measurement

Lipoprotein lipase (LPL) is a lipolytic enzyme involved in catalyzing hydrolysis of triglycerides (TG) in chylomicrons and very low-density lipoprotein (VLDL) particles. Over the last decade, increasing attention has been paid to the clinical significance of measuring serum LPL protein mass without heparin injection to the study subjects. In earlier studies, this marker was utilized to classify LPL deficient subjects, which is an extremely rare metabolic disorder with a frequency of one in one million. Later, researchers paid more attention to the clinical significance of measuring this parameter in more common metabolic disorders. Studies have shown that pre-heparin plasma or serum LPL mass has significant relationships with serum lipids and lipoproteins, visceral fat area, insulin resistance, and even the development of coronary atherosclerosis in cross-sectional studies, although this might be a metabolic surrogate marker with almost no catalytic activities, which does not appear to be involved in catalyzing hydrolysis of TG in TG-rich lipoproteins. Recently, a prospective study has demonstrated that low serum LPL concentration predicts future coronary events. Taken together, we suggest that pre-heparin LPL mass in plasma or sera provide us with useful and important information on the development of metabolic disorders leading to atherosclerotic disease.

Lipoprotein-associated proteins involved in platelet signaling

Platelets and lipoproteins are both key elements in the development of atherosclerosis and thrombosis. Based on their density, five classes of lipoproteins have been identified which all influence platelets via distinct mechanisms. The activation of platelets starts with binding of apolipoproteins to different platelet receptors and is followed by the activation of signaling pathways resulting in activation or inhibition of platelet functions like aggregation or secretion. In addition to apolipoproteins, lipoproteins are also associated to a large amount of proteins, enzymes and lipids that also can induce platelet activation or inhibition. This review provides a summary of the activation of signaling pathways after platelet-lipoprotein interactions initiated by lipoprotein-associated proteins and lipids.

Localized vessel expression of lipoprotein lipase in rabbits leads to rapid lipid deposition in the balloon-injured arterial wall

Recent studies on mice demonstrated that lipoprotein lipase (LPL) located in the arterial wall might play a pro-atherogenic role. There are major differences between humans and mice in lipoprotein metabolism and in susceptibility to atherosclerosis. We have therefore used rabbits fed normal chow diet as a model to assess such localized effects by adenovirus-mediated gene transfer of human catalytically active wild type LPL (hLPLwt) and an inactive mutant (hLPL194) to balloon-injured carotid arteries. By morphometric analysis on cryosections stained with Oil Red O (ORO) we found 7- and 4-fold increases, respectively, of lipid deposition in the arterial walls 7 days after infection with adenovirus expressing hLPLwt or hLPL194, when compared with a virus expressing human alkaline phosphatase (hAP) as control. Macrophages were detected in the arteries expressing both forms of LPL, but apoB was only found in arteries expressing hLPLwt. Expression of the LPL gene products was transient and was gone after 2 weeks, but the accumulated lipid deposits remained between the neointimal and the media layers even after 8 weeks. Our data demonstrate that expression of LPL in the arterial wall (with or without lipase activity) leads to lipid accumulation in balloon-injured rabbit arteries, and could result in enhanced formation of atherosclerotic lesions.

Fat oxidation and plasma removal capacity of an intravenous fat emulsion in elderly and young men

OBJECTIVE

We explored metabolic and thermogenic responses to exogenous fat in relation to age as a basis for a rational design of parenteral nutrition in elderly patients.

METHODS

Ten healthy elderly men (70-78 y of age, body mass index 21-27 kg/m(2)) and 10 healthy young men (19-45 y of age, body mass index 19-26 kg/m(2)) were studied with a hypertriglyceridemic clamp (primed infusion of a long-chain triacylglycerol emulsion to reach and stabilize at a triacylglycerol concentration of 4 mmol/L for 180 min). Continuous indirect calorimetry was carried out in the basal state and throughout the study period.

RESULTS

The infusion rates required to maintain plasma triacylglycerol levels at 4 mmol/L were similar in elderly and young individuals (mean +/- SEM 0.201 +/- 0.027 versus 0.203 +/- 0.014 mmol/min, not significant). Plasma concentrations of free fatty acids and beta-OH-butyrate were higher in the elderly before the infusion and increased in a similar manner in both groups during infusion. Energy expenditure at baseline was higher in the young than in the elderly (79 +/- 2 versus 64 +/- 3 kcal/h; P < 0.001), although the respiratory quotient was similar in the two groups (0.80 +/- 0.01 versus 0.78 +/- 0.01, not significant). During lipid administration there was a similar increase in energy expenditure in the old and young individuals (+9.0 +/- 1.3% versus +6.0 +/- 1.8%, not significant). Lipid infusion resulted in similar increments in fat oxidation in the young and elderly (23.9 +/- 7.0% versus 15.1 +/- 4.9%, respectively, not significant). Plasma lipoprotein lipase activity was almost three times higher in the young than in the elderly subjects (0.23 +/- 0.02 versus 0.65 +/- 0.09 mU/mL, respectively, P < 0.001). During lipid infusion, a similar increment (four- to five-fold) in plasma lipoprotein lipase activity was noted in the two groups.

CONCLUSIONS

Elderly healthy men have a similar capacity as young healthy men to clear and oxidize a high triacylglycerol load administered as a hypertriglyceridemic clamp.

Manipulation of inflammation modulates hyperlipidemia in apolipoprotein E-deficient mice: a possible role for interleukin-6

There are increasing evidences showing that inflammation participates in atherosclerosis. Therefore, the therapeutic use of anti-inflammatory agents should be considered. We have induced chronic, aseptic inflammation upon the injection of turpentine and tested the effect of dexamethasone on lipoprotein metabolism and, consequently, atherosclerosis in apolipoprotein E-deficient mice. Aseptic inflammation caused a significant decrease in hyperlipidemia. Treatment with dexamethasone elicited the opposite effect increasing hyperlipidemia through mechanisms related to the increase in the synthesis of triglyceride-rich lipoproteins. Changes in plasma lipids correlated with those observed in the size of atherosclerotic lesions. Our data suggest the presence of a common mechanism present in both observations and which is probably related to the cytokine secretion. Among the candidates, we chose to test the effect of interleukin-6 because it is involved in both processes, atherosclerosis and inflammation, and its expression is efficiently repressed by corticosteroids. The injection of recombinant interleukin-6 in our mice elicited the same effects observed in our model of inflammation. We conclude that manipulation of inflammation-related mechanisms modulates lipid homeostasis and development of atherosclerotic plaque in rodents.

Lipoprotein lipase S447X: a naturally occurring gain-of-function mutation

Lipoprotein lipase (LPL) hydrolyzes triglycerides in the circulation and promotes the hepatic uptake of remnant lipoproteins. Since the gene was cloned in 1989, more than 100 LPL gene mutations have been identified, the majority of which cause loss of enzymatic function. In contrast to this, the naturally occurring LPL(S447X) variant is associated with increased lipolytic function and an anti-atherogenic lipid profile and can therefore be regarded as a gain-of-function mutation. This notion combined with the facts that 20% of the general population carries this prematurely truncated LPL and that it may protect against cardiovascular disease has led to extensive clinical and basic research into this frequent LPL mutant. It is only until recently that we begin to understand the molecular mechanisms that underlie the beneficial effects associated with LPL(S447X). This review summarizes the current literature on this interesting LPL variant.

Serum lipoprotein lipase concentration and risk for future coronary artery disease: the EPIC-Norfolk prospective population study

BACKGROUND

Lipoprotein lipase (LPL) is associated with coronary artery disease (CAD) risk, but prospective population data are lacking. This is mainly because of the need for cumbersome heparin injections, which are necessary for LPL measurements. Recent retrospective studies, however, indicate that LPL concentration can be reliably measured in serum that enabled evaluation of the prospective association between LPL and future CAD.

METHODS AND RESULTS

LPL concentration was determined in serum samples of men and women in the EPIC-Norfolk population cohort who developed fatal or nonfatal CAD during 7 years of follow-up. For each case (n=1006), 2 controls, matched for age, sex, and enrollment time, were identified. Serum LPL concentration was lower in cases compared with controls (median and interquartile range: 61 [43-85] versus 66 [46-92] ng/mL; P<0.0001). Those in the highest LPL concentration quartile had a 34% lower risk for future CAD compared with those in the lowest quartile (odds ratio [OR] 0.66; confidence interval [CI], 0.53 to 0.83; P<0.0001). This effect remained significant after adjustment for blood pressure, diabetes, smoking, body mass index, and low-density lipoprotein (LDL) cholesterol (OR, 0.77; CI, 0.60-0.99; P=0.02). As expected from LPL biology, additional adjustments for either high-density lipoprotein cholesterol (HDL-C) or triglyceride (TG) levels rendered loss of statistical significance. Of interest, serum LPL concentration was positively linear correlated with HDL and LDL size.

CONCLUSIONS

Reduced levels of serum LPL are associated with an increased risk for future CAD. The data suggest that high LPL concentrations may be atheroprotective through decreasing TG levels and increasing HDL-C levels.

Dietary cholesterol and differential monocyte chemoattractant protein-1 gene expression in aorta and liver of apo E-deficient mice

In humans, hypercholesterolemia, steatohepatitis, and risk for arteriosclerosis are associated. Apolipoprotein E-deficient mice, a widely used animal model, show both arteriosclerosis and steatohepatitis in response to high-fat and cholesterol diets. We have found a relationship between these conditions and a higher mRNA aortic and hepatic monocyte chemoattractant protein-1 (mcp-1) gene expression. Both tissues respond in a similar way when dietary cholesterol is provided for a few weeks but differently if the conditions persist for a protracted period of time. After 8 months of treatment, the mcp-1 gene expression in the aorta continues increasing but in the liver decreases. This coincides with a significant increase in hepatic ppar-delta anti-inflammatory gene expression. Apparently, the arterial wall cannot prevent the deleterious effects of higher mcp-1 expression by increasing ppar-delta gene expression and the lesion progress. However, in the liver, the activation of anti-inflammatory genes may reduce the hepatic mcp-1 expression which significantly decreases the inflammatory response. This differential inflammatory gene expression in aorta and liver may support the idea that anti-inflammatory transcription factors are involved in the response to diet and inflammation. Therefore, the use of cholesterol-enriched diets should be carefully considered in the apolipoprotein E-deficient mice because they may trigger different stimuli and seriously hinder the interpretation of possible findings.

Enhanced Conversion of Triglyceride-Rich Lipoproteins and Increased Low-Density Lipoprotein Removal in LPLS447X Carriers

OBJECTIVE

Lipoprotein lipase (LPL) exerts 2 principal actions, comprising enzymatic hydrolysis of triglyceride-rich lipoproteins (TRLs) and nonenzymatic ligand capacity for enhancing lipoprotein removal. The common LPLS447X variant has been associated with cardiovascular protection, for which the mechanism is unknown. We therefore evaluated enzymatic and nonenzymatic consequences of this LPL variant on TRL metabolism.

METHODS AND RESULTS

TRL apolipoprotein B100 (apoB100) metabolism was determined in 5 homozygous LPLS447X carriers and 5 controls. Subjects were continuously fed and received infusion of stable isotope l-[1-(13C)]-valine. Results were analyzed by SAAMII modeling. Also, preheparin and postheparin LPL concentration and activity were measured. Compared with controls, carriers presented increased very low-density lipoprotein 1 (VLDL1) to VLDL2 apoB100 flux (P=0.04), increased VLDL2 to intermediate-density lipoprotein (IDL) apoB100 flux (P=0.02), increased IDL to low-density lipoprotein (LDL) apoB100 flux (P=0.049), as well as an increased LDL clearance (P=0.04). Additionally, IDL apoB100 synthesis was attenuated (P=0.05). Preheparin LPL concentration was 4-fold higher compared with controls (P=0.01), and a correlation was observed between preheparin LPL concentration and LDL clearance (r2=0.92; P=0.01).

CONCLUSIONS

Enhanced TRL conversion and enhanced LDL removal combined with increased preheparin LPL concentration suggest increased enzymatic consequences as well as increased nonenzymatic consequences of LPL in LPLS447X carriers, which might both contribute to the cardiovascular benefit of this LPL variant.

Carriers of the frequent lipoprotein lipase S447X variant exhibit enhanced postprandial apoprotein B-48 clearance

The frequent lipoprotein lipase S447X variant (LPLS447X) is firmly associated with a lower incidence of cardiovascular disease, the mechanisms for which remain to be established. To further unravel these beneficial effects, we studied the consequences of LPLS447X heterozygosity on LPL mass and activity, as well as on the postprandial lipoprotein profile. Fifteen male heterozygous LPLS447X carriers and 15 matched control subjects received an oral fat load (30 g/m(2)). Lipid parameters were evaluated at baseline and 2, 3, 4, and 6 hours after fat loading. LPL concentration and activity were analyzed, and endothelial function was evaluated noninvasively as flow-mediated dilation of the brachial artery. Although baseline apoprotein B-48 (apoB48) levels were similar, the rise in apoB48 was attenuated in LPLS447X carriers with 25% lower peak values compared with controls (P=.04). In conjunction, LPLS447X carriers were characterized by a 2.4-fold increase in pre-heparin LPL mass (P<.0001). The decrease in postprandial flow-mediated dilation was comparable in both groups. LPLS447X carriers exhibit enhanced apoB48 clearance with concomitant increase in pre-heparin LPL mass, without changes in LPL activity. This combination might suggest a role for increased ligand action of LPL in LPLS447X carriers contributing to the cardiovascular protection in carriers of this mutation.

Low density lipoprotein non-esterified fatty acids and lipoprotein lipase in diabetes

OBJECTIVES

Fatty acid metabolism is disturbed in poorly controlled diabetes. Low density lipoprotein (LDL) oxidation, thought to be an atherogenic modification, is partly dependent on LDL fatty acid content whether it be in the form of cholesteryl ester, phospholipids, triglyceride or non-esterified fatty acid (NEFA). Lipoprotein lipase (LPL) is deficient in diabetic patients. Lipoprotein lipase bound to LDL may facilitate cholesterol accumulation in the artery wall through the attachment of LDL to the proteoglycans expressed on endothelial cells and collagen. The purpose of this study was to examine the degree of binding of fatty acids and lipoprotein lipase to LDL in type 2 diabetic patients and to examine the relationship between non-esterified fatty acids attached to LDL and LDL oxidisability.

SUBJECTS AND METHODS

Eight type 2 diabetic patients and eight control subjects were examined fasting and at 4 and 6h following a high fat meal. Six control subjects were examined fasting and 30 min after intravenous heparin. LDL was isolated by sequential ultracentrifugation. Individual LDL non-esterified fatty acids were measured by gas-liquid chromatography following transmethylation. LPL and oxidised LDL were measured by ELISA.

RESULTS

The diabetic patients had HbA1c of 7.8 +/- 0.5% confirming moderate diabetic control. There was a large increase in the mean non-esterified fatty acids on LDL from diabetic subjects (0.66 +/- 0.40 mg/mg versus 0.06 +/- 0.02 mg/mg LDL protein, p < 0.01). Mean LDL cholesterol ester fatty acids were also significantly increased in the diabetic subjects (1.47 +/- 0.58 mg/mg versus 0.57 +/- 0.40 mg/mg LDL protein, p < 0.01). There was a significant increase in oxidised LDL (31.2 +/- 24 mg/mg versus 7.7 +/- 4.5 mg/mg LDL protein, p < 0.01) and a significant correlation between postprandial non-esterified fatty acid and LDL oxidation (r = 0.69, p < 0.05). LPL was significantly increased on the LDL but not in the plasma of diabetic subjects. Acute elevation in non-esterified fatty acids produced by heparin in control subjects did not increase LDL non-esterified fatty acids.

CONCLUSIONS

This study demonstrates that the disturbance in fatty acid metabolism found in type 2 diabetic subjects is associated with a significant increase in non-esterified fatty acids attached to LDL. This may account, at least in part, for the increased oxidation of the LDL and therefore its atherogenicity. The finding of an increase in the amount of LPL bound to LDL suggests an important mechanism to facilitate the uptake of diabetic LDL by endothelial proteoglycans and collagen in the atherosclerotic plaque.

Feeding apolipoprotein E-knockout mice with cholesterol and fat enriched diets may be a model of non-alcoholic steatohepatitis

The present study was aimed (1) to investigate the effect of cholesterol and fat enriched diets on the development of steatohepatitis in apolipoprotein E-knockout mice, and (2) to study the chronological relationships between the development of hepatic alterations, hypercholesterolemia and atherosclerotic lesions in this experimental model. The study consisted of two protocols. Protocol 1 was used in 90 mice subdivided in groups of 18. For 10 weeks, each group was given a diet with different fat and cholesterol contents. Protocol 2 was used in 42 mice, subdivided in four groups. Each group was given a diet enriched with cholesterol and palm oil and they were sacrificed at 8, 13, 18 and 24 weeks of age. Results were as following. (1) Mice given high fat/high cholesterol diets developed an impairment of liver histology consisting of fat accumulation, macrophage proliferation, and inflammation. (2) These effects were modulated by the type of fat: olive oil was mainly associated with macrovesicular steatosis and cholesterol plus palm oil with severe steatohepatitis. (3) There was a chronological and quantitative relationship between liver impairment and the formation of atheromatous lesions. We conclude that apolipoprotein E-knockout mice may be a useful model for investigating the mechanisms of diet-induced steatohepatitis.

Changes in lipoprotein lipase modulate tissue energy supply during stress

We studied the variations caused by stress in lipoprotein lipase (LPL) activity, LPL-mRNA, and local blood flow in LPL-rich tissues in the rat. Stress was produced by body immobilization (Immo): the rat's limbs were taped to metal mounts, and its head was placed in a plastic tube. Chronic stress (2 h daily of Immo) decreased total LPL activity in mesenteric and epididymal white adipose tissue (WAT) and was accompanied by a weight reduction of these tissues. In limb muscle, heart, and adrenals, total LPL activity and mRNA levels increased, and, in plasma, LPL activity and mass also increased. Acute stress (30-min Immo) caused a decrease in total LPL activity only in retroperitoneal WAT and an increase in preheparin plasma active LPL, but the overall weight of this tissue did not vary significantly. We propose an early release of the enzyme from this tissue into the bloodstream by some unknown extracellular pathways or other local mechanisms. These changes in this key energy-regulating enzyme are probably induced by catecholamines. They modify the flow of energy substrates between tissues, switching the WAT from importer to exporter of free fatty acids and favoring the uptake by muscle of circulating triacylglycerides for energy supply. Moreover, we found that acute stress almost doubled blood flow in all WAT studied, favoring the export of free fatty acids.

Turpentine-induced inflammation reduces the hepatic expression of the multiple drug resistance gene, the plasma cholesterol concentration and the development of atherosclerosis in apolipoprotein E deficient mice

We aimed to investigate the effect of turpentine-induced inflammation in an atherosclerosis-prone murine model. We have induced a chronic aseptic inflammation in apolipoprotein E-deficient mice, with or without a dietary supplement of aspirin (n = 10, each), by the injection of a mixture (1:1) of turpentine and olive oil in the hind limb twice weekly for a period of 12 weeks. Control animals were injected with olive oil alone (n = 10). The control mice did show any alteration neither in plasma nor at the site of injection. Turpentine-treated mice showed a significant increase in plasma TNF-alpha and SAA concentrations which indicated a systemic inflammatory response that was not substantially affected by aspirin. Also, turpentine injections significantly reduced the plasma cholesterol concentration, probably decreasing intestinal cholesterol re-absorption, and attenuated the size of atherosclerotic lesion. Both effects were minimally influenced by aspirin. The burden of atherosclerosis correlated with plasma lipid levels but not with plasma inflammatory markers. Finally, there was a concomitant decrease in the expression of the hepatic mdr1b gene that correlated with the decrease in plasma cholesterol concentration. Therefore, we conclude that mdr1 is an additional factor to consider in the complexity of alterations in cholesterol metabolism that occur in this model.

Lipoprotein lipase-dependent binding and uptake of low density lipoproteins by THP-1 monocytes and macrophages: possible involvement of lipid rafts

Lipoprotein lipase (LPL) is produced by cells in the artery wall and can mediate binding of lipoproteins to cell surface heparan sulfate proteoglycans (HSPG), resulting in endocytosis (the bridging function). Active, dimeric LPL may dissociate to inactive monomers, the main form found in plasma. We have studied binding/internalization of human low density lipoprotein (LDL), mediated by bovine LPL, using THP-1 monocytes and macrophages. Uptake of (125)I-LDL was similar in monocytes and macrophages and was not affected by the LDL-receptor family antagonist receptor-associated protein (RAP) or by the phagocytosis inhibitor cytochalasin D. In contrast, uptake depended on HSPG and on membrane cholesterol. Incubation in the presence of dexamethasone increased the endogenous production of LPL by the cells and also increased LPL-mediated binding of LDL to the cell surfaces. Monomeric LPL was bound to the cells mostly in a heparin-resistant fashion. We conclude that the uptake of LDL mediated by LPL dimers is receptor-independent and involves cholesterol-enriched membrane areas (lipid rafts). Dimeric and monomeric LPL differ in their ability to mediate binding/uptake of LDL, probably due to different mechanisms for binding/internalization.

Anti-lipoprotein lipase antibodies: A new player in the complex atherosclerotic process in systemic lupus erythematosus?

OBJECTIVE

The novel description of antibodies to lipoprotein lipase (anti-LPL) associated with dyslipoproteinemia prompted us to analyze the association of anti-LPL with clinical and serologic features in patients with systemic lupus erythematosus (SLE) and its link to markers of inflammation that are known to be involved in atherogenesis.

METHODS

Enzyme-linked immunosorbent assay was used to test for the presence of anti-LPL antibodies in 66 consecutive patients with SLE. Clinical and laboratory evaluation, including a fasting lipid profile, autoantibody screening, an assessment for markers of inflammation (C-reactive protein [CRP], erythrocyte sedimentation rate [ESR]), and the SLE Disease Activity Index (SLEDAI) were performed at the time of inclusion in the study. Exclusion criteria were any conditions that affect the lipid profile. SLE patients were categorized into 2 groups according to detection of these anti-LPL antibodies, as follows: anti-LPL+ and anti-LPL-.

RESULTS

Anti-LPL antibody IgG was detected in 25 SLE patients (37.8%). Triglyceride levels were significantly higher in the anti-LPL+ group (112.4 +/- 50.2 versus 89.9 +/- 54.5 mg/dl in the anti-LPL- group; P = 0.033), but no significant differences between the 2 groups were detected for total, high-density lipoprotein, and low-density lipoprotein cholesterol levels. A higher frequency of elevated CRP levels and ESRs was observed in the anti-LPL+ group compared with the anti-LPL- group (44% and 17.1%, respectively [P = 0.023] and 52% and 19.5%, respectively [P = 0.013]). Moreover, SLE patients with anti-LPL antibodies also had significantly higher levels of CRP (11.1 +/- 16.4 versus 2.4 +/- 2.6 mug/ml; P = 0.036) and higher ESRs (33.4 +/- 29.8 versus 16.5 +/- 11.8 mm/hour; P = 0.020). Anti-LPL titers had a significant positive correlation with the CRP level (r = 0.56, P < 0.001), the ESR (r = 0.55, P < 0.001), the SLEDAI score (r = 0.45, P < 0.001), anti-double-stranded DNA (anti-dsDNA; r = 0.52, P < 0.001), and anticardiolipin IgG antibodies (r = 0.25, P = 0.04), and a significant negative correlation was detected with total hemolytic complement activity (CH100) (r = -0.34, P = 0.005). Reinforcing these findings, multiple regression analysis also revealed a significant association of anti-LPL with the CRP level (P = 0.025) and anti-dsDNA (P < 0.001). Importantly, a comparison of positive and negative anti-dsDNA sera revealed similar mean CRP levels (P = 0.56) and ESRs (P = 0.102), contrasting with the SLEDAI score (P = 0.004) and CH100 (P = 0.008).

CONCLUSION

These data support the link between inflammation, immune response, and dyslipoproteinemia in SLE, introducing anti-LPL as a possible new player that may ultimately help in understanding the complex events of atherogenesis in this disease.

Rapid subunit exchange in dimeric lipoprotein lipase and properties of the inactive monomer

Lipoprotein lipase (LPL), a key enzyme in the metabolism of triglyceride-rich plasma lipoproteins, is a homodimer. Dissociation to monomers leads to loss of activity. Evidence that LPL dimers rapidly exchange subunits was demonstrated by fluorescence resonance energy transfer between lipase subunits labeled with Oregon Green and tetrametylrhodamine, respectively, and also by formation of heterodimers composed of radiolabeled and biotinylated lipase subunits captured on streptavidine-agarose. Compartmental modeling of the inactivation kinetics confirmed that rapid subunit exchange must occur. Studies of activity loss indicated the existence of a monomer that can form catalytically active dimers, but this intermediate state has not been possible to isolate and remains hypothetical. Differences in solution properties and conformation between the stable but catalytically inactive monomeric form of LPL and the active dimers were studied by static light scattering, intrinsic fluorescence, and probing with 4,4'-dianilino-1,1'-binaphtyl-5,5'-disulfonic acid and acrylamide. The catalytically inactive monomer appeared to have a more flexible and exposed structure than the dimers and to be more prone to aggregation. By limited proteolysis the conformational changes accompanying dissociation of the dimers to inactive monomers were localized mainly to the central part of the subunit, probably corresponding to the region for subunit interaction.

Lipoprotein lipase (LPL) mass in preheparin serum reflects insulin sensitivity

Lipoprotein lipase (LPL) is one of the enzymes regulated by insulin and its plasma activity reflects insulin sensitivity. Although intravenous heparin injection is required to measure LPL activity, we can detect LPL mass in preheparin serum (Pr-LPL mass) by immunoassay. In this study, we examined whether Pr-LPL mass reflects insulin sensitivity. We measured Pr-LPL mass, insulin sensitivity (Si), and acute insulin release in response to a glucose bolus (AIRg) in subjects with normal glucose tolerance (NGT; n = 23), impaired glucose tolerance (IGT; n = 10), and Type II diabetes mellitus (DM; n = 48). Si and AIRg were determined by minimal model analysis. We also compared Pr-LPL mass with the homeostasis model assessment of insulin resistance (HOMA-R) and the urinary excretion of C-peptide (urine CPR). We found that Pr-LPL mass correlated significantly with Si ( r = 0.354, P < 0.01) in all the subjects. This correlation was still significant in the NGT group (P < 0.472, P < 0.05), DM group (r = 0.311, P < 0.01), and DM group with a fasting plasma glucose >150 mg/dl ( n = 20, r = 0.459. P < 0.05). Moreover, Pr-LPL mass correlated negatively with HOMA-R (r = -0.272. P < 0.05) and fasting IRI (r = -0.256, P < 0.05). By contrast, Pr-LPL mass was not correlated with either urine CPR or logAIRg that reflect the ability to secrete insulin. In conclusion, Pr-LPL mass reflects insulin sensitivity. We speculate that Pr-LPL mass might be used to assess insulin sensitivity not only in the general population but also in advanced diabetic patients.

Lipoprotein lipase in the kidney: activity varies widely among animal species

Much evidence points to a relationship among kidney disease, lipoprotein metabolism, and the enzyme lipoprotein lipase (LPL), but there is little information on LPL in the kidney. The range of LPL activity in the kidney in five species differed by >500-fold. The highest activity was in mink, followed by mice, Chinese hamsters, and rats, whereas the activity was low in guinea pigs. In contrast, the ranges for LPL activities in heart and adipose tissue were less than six- and fourfold, respectively. The activity in the kidney (in mice) decreased by >50% on food deprivation for 6 h without corresponding changes in mRNA or mass. This decrease in LPL activity did not occur when transcription was blocked with actinomycin D. Immunostaining for kidney LPL in mice and mink indicated that the enzyme is produced in tubular epithelial cells. To explore the previously suggested possibility that the negatively charged glomerular filter picks up LPL from the blood, bovine LPL was injected into rats and mice. This resulted in decoration of the glomerular capillary network with LPL. This study shows that in some species LPL is produced in the kidney and is subject to nutritional regulation by a posttranscriptional mechanism. In addition, LPL can be picked up from blood in the glomerulus.

Aspirin Attenuates the Initiation but Not the Progression of Atherosclerosis in Apolipoprotein E-Deficient Mice Fed a High-Fat, High-Cholesterol Diet

Aspirin has potent antiinflammatory properties and attenuates atherosclerosis in apolipoprotein-E-deficient mice fed a high-fat, high-cholesterol diet. In an attempt to clarify the contradictory results obtained with normal chow, we studied the effect of aspirin for a prolonged period of time. The mice were fed a commercial chow until the experiment began at 8 weeks of age. Blood samples were then obtained and several mice (n=8) were sacrificed. The diet of the remaining 48 animals was supplemented with 200 g/kg palm fat and 1 g/kg cholesterol. They were then randomly divided into 2 groups, one of which received 0.5 mg/day of aspirin. The aspirin had a time-dependent effect. First, the extent of lesion decreased; then the effect was neutral; and, finally, after longer periods of being fed the atherogenic diet and receiving aspirin, the extent of the lesion increased. The transitory effect of aspirin should be elucidated in the absence of high dietary lipids.

Triacylglycerol-rich lipoprotein margination: a potential surrogate for whole-body lipoprotein lipase activity and effects of eicosapentaenoic and docosahexaenoic acids

BACKGROUND

Margination occurs when blood borne particles attach to the vessel wall. Triacylglycerol-rich lipoprotein (TRL) particles marginate when they bind to endothelial lipoprotein lipase (LpL).

OBJECTIVE

This study was undertaken to determine whether TRL margination reflects in vivo LpL activity and whether n-3 fatty acids affect fasting and fed TRL margination.

DESIGN

Healthy subjects (n = 33) began with a 4-wk, placebo (olive oil; 4 g/d) run-in period and were then randomly assigned to 4 wk of treatment with 4 g/d of ethyl esters of either safflower oil (SAF; control), eicosapentaenoic acid (EPA), or docosahexaenoic acid (DHA). Margination volume (MV) was calculated by subtracting true from apparent plasma volume.

RESULTS

MVs were 3 times as great during the fasting state as during the fed state (P < 0.0001). In both the fasting and the fed states, MV was significantly correlated with plasma triacylglycerol and TRL half-lives. In the fed state, MV was also correlated with preheparin LpL, whereas in the fasting state it was not. There was no significant correlation between preheparin LpL and postheparin LpL in the fasting state. Relative to SAF, EPA and DHA supplementation resulted in higher MVs by 64% and 53% (both P < 0.001), respectively, in the fasting state, without significantly reducing fasting triacylglycerol concentrations. In the fed state, DHA doubled the MV (P < 0.05), whereas EPA had no significant effect.

CONCLUSIONS

The correlations between MV and TRL half-lives and preheparin LpL suggest that MV could be a reflection of whole-body LpL binding capacity. The increases in MVs with EPA and DHA supplementation suggest that these fatty acids may increase the amount of endothelial-bound LpL or its affinity for TRL.

Measurement of the serum lipoprotein lipase concentration is useful for studying triglyceride metabolism: Comparison with postheparin plasma

The catalytically inactive form of lipoprotein lipase (LPL) is detectable at high levels in serum, although its physiologic role remains unknown. The aim of this study was to elucidate the clinical significance of serum LPL compared with postheparin LPL or the net increment (Delta) of LPL (postheparin - preheparin LPL). We measured the LPL mass before and 15 minutes after the injection of heparin in 164 subjects with hyperlipidemia. LPL mass was measured by a sensitive sandwich enzyme-linked immunosorbent assay (ELISA). Serum LPL was one fifth of the postheparin LPL concentration. There was a weak correlation between the serum LPL and postheparin LPL concentrations (r =.225, P </=.005). The Delta LPL concentration was strongly related to the postheparin LPL concentration (r =.965, P </=.0001), but not to the preheparin LPL mass, suggesting that the weak correlation between serum LPL and postheparin LPL levels was attributable to contamination of postheparin plasma by pre-existing LPL (preheparin LPL). Both serum and postheparin LPL were significantly lower in diabetic patients and in subjects with high levels of triglyceride or low levels of high-density lipoprotein (HDL). Serum LPL was correlated negatively with triglyceride, remnants, and insulin resistance and was positively correlated with HDL cholesterol and low-density lipoprotein (LDL) size. Postheparin LPL was strongly correlated with HDL cholesterol, but not with other parameters, as was serum LPL. Delta LPL mass did not show a closer association with triglyceride metabolism than postheparin LPL or preheparin LPL. In conclusion, serum LPL measurement is simple and seems to be useful for studying triglyceride metabolism.

Alteration in lipoprotein lipase activity bound to triglyceride-rich lipoproteins in the postprandial state in type 2 diabetes

Postprandial lipid metabolism is largely dependent upon lipoprotein lipase (LPL), which hydrolyses triglycerides (TGs). The time course of LPL activity in the postprandial state following a single meal has never been studied, because its determination required heparin injection. Recently, we have shown that LPL activity could be accurately measured in nonheparinized VLDL using a new assay aiming to determine the LPL-dependent VLDL-TG hydrolysis. Based on the same principle, we used in this study TG-rich lipoprotein (TRL)-bound LPL-dependent TRL-TG hydrolysis (LTTH) to compare the time course of LPL activity of 10 type 2 diabetics to that of 10 controls, following the ingestion of a lipid-rich meal. The peak TG concentration, reached after 4 h, was 67% higher in diabetics than in controls (P < 0.005). Fasting LTTHs were 91.3 +/- 15.6 in controls versus 70.1 +/- 4.8 nmol NEFA/ml/h in diabetics (P < 0.001). LTTH was increased 2 h postprandially by 190% in controls and by only 89% in diabetics, resulting in a 35% lowering of the LTTH area under the curve in diabetics. Postprandial LTTH was inversely correlated with TG or remnant concentrations in controls but not in diabetics, and with insulin resistance in both groups. These data show that TRL-bound LPL activity increases in the postprandial state and is strongly reduced in type 2 diabetes, contributing to postprandial hypertriglyceridemia.

Pre-heparin Lipoprotein Lipase Mass

Lipoprotein lipase (LPL) is a lipolytic enzyme involved in catalyzing the hydrolysis of triglycerides (TG) in chylomicrons and very low-density lipoprotein (VLDL) particles. Over the last decade, the clinical significance of measuring LPL mass without heparin injection has been increasingly studied. In earlier studies, it was shown that this marker was utilized to classify type 1 hyperlipoproteinemia, which is an extremely rare metabolic disorder. Later, researchers paid more attention to the clinical significance of measuring this parameter in more common metabolic disorders. Studies have shown that pre-heparin plasma LPL mass has significant relationships with serum lipid and lipoproteins, visceral fat area, and even a marker for acute inflammation, although this might be a metabolic surrogate marker which does not appear to be involved in catalyzing the hydrolysis of TG in TG-rich lipoproteins. We suggest that pre-heparin LPL mass in plasma or sera provides us with useful and important information on the pathophysiology of metabolic disorders or acute inflammation despite its simplicity from a practical point of view.

Circulating blood cells modulate the atherosclerotic process in apolipoprotein E-deficient mice

The interaction of blood with the arterial tree may play an important role in the development of atherosclerotic lesions. The aims of this study were (1) to determine how anemia or increased hematocrit affect the development of atherosclerosis and (2) to find relationships between hematologic and hemorrheologic variables in apolipoprotein (apo) E-deficient mice. Forty-two mice were randomly divided into 3 groups of 14 mice each. There was no further manipulation in the control group. To induce anemia, the mice from one of the groups were repeatedly bled, drawing approximately 250 microL blood from each mouse twice a week. To increase the hematocrit levels in another group of mice, we injected 20 U recombinant human erythropoietin every other day. The development of lesions and the main variables involved in atherogenesis were compared among groups. Our results show that atherosclerosis was attenuated in the mice that were bled, and this was not accounted for by changes in plasma lipid levels, the distribution of lipoprotein particles, the body iron distribution, or oxidation parameters. Moreover, atherosclerosis was enhanced in the mice treated with the continuous administration of erythropoietin. To ascertain the relationship between hematocrit and whole blood viscosity, we measured both variables in pooled blood from 24 additional mice, which were manipulated to ensure a wide range of values. We found a direct and significant correlation between hematocrit and blood viscosity and between hematocrit and lesion size. Our data support in vivo the idea that hemorrheology has an important role in atherogenesis in this particular animal model.