Human hepatic triglyceride lipase expression reduces high density lipoprotein and aortic cholesterol in cholesterol-fed transgenic mice

Abnormalities in apo B-containing lipoproteins in diabetes and atherosclerosis

Atherosclerosis is the major cause of death in patients with diabetes. Low-density lipoprotein (LDL) being the most important cholesterol-carrying lipoprotein has been studied extensively in both diabetes and non-diabetes. This paper reviews the literature but also focuses on the precursors of LDL and in particular the postprandial apo B-containing lipoproteins. Abnormalities in the postprandial lipoproteins and alteration in chylomicron assembly and clearance are discussed and the evidence presented suggesting the importance of dysregulation of these lipoproteins in atherosclerotic progression. The relationship between chylomicron production in the intestine and hepatic release of very low-density lipoproteins (VLDL) is explored, as is the interrelationship between clearance rates of these lipoproteins. The size of LDL influences its atherogenicity. VLDL composition and size in relation to its influence on LDL is discussed. The effect of diet on the composition of lipoproteins and the relationship between fatty acid composition and clearance is reviewed. Evidence that diabetic control beneficially alters lipoprotein composition is presented suggesting how improved diabetic control may reduce atherosclerosis. The review concludes with a discussion on the effect of the apo B-containing lipoproteins and their modification through glycation and oxidation on macrophage and endothelial function.

Molecular mechanisms, lipoprotein abnormalities and atherogenicity of hyperalphalipoproteinemia

Hyperalphalipoproteinemia (HALP) is caused by a variety of genetic and environmental factors. Among these, plasma cholesteryl ester transfer protein (CETP) deficiency is the most important and frequent cause of HALP in the Asian populations. CETP facilitates the transfer of cholesteryl ester (CE) from high density lipoprotein (HDL) to apolipoprotein (apo) B-containing lipoproteins, and is a key protein in the reverse cholesterol transport system. The deficiency of CETP causes various abnormalities in the concentration, composition, and function of both HDL and low density lipoprotein (LDL). The significance of CETP in terms of atherosclerosis had been controversial. However, the in vitro evidence showed large CE-rich HDL particles in CETP deficiency are defective in cholesterol efflux. Similarly, scavenger receptor BI (SR-BI) knockout mice show a marked increase in HDL-cholesterol but accelerated atherosclerosis in atherosclerosis-susceptible mice. Recent epidemiological studies in Japanese-Americans and in Omagari area where HALP subjects with the intron 14 splicing defect of CETP gene are markedly frequent, have demonstrated an increased incidence of coronary atherosclerosis in CETP-deficient patients. Thus, CETP deficiency is a state of impaired reverse cholesterol transport which may possibly lead to the development of atherosclerosis. The current review will focus on the molecular mechanisms and atherogenicity of HALP, especially CETP deficiency.

High-density lipoprotein metabolism: molecular targets for new therapies for atherosclerosis

New therapeutic approaches to the prevention and treatment of atherosclerotic cardiovascular disease (ASCVD) are needed. Plasma levels of high-density lipoprotein (HDL) cholesterol are inversely associated with risk of ASCVD. Genes involved in the metabolism of HDL represent potential targets for the development of such therapies. Because HDL metabolism is a dynamic process, the effect of a specific HDL-oriented intervention on atherosclerosis cannot necessarily be predicted by its effect on the plasma HDL cholesterol level. Based on available data in animal models, some gene products are candidates for pharmacologic upregulation, infusion, or overexpression, including apolipoprotein (apo)A-I, apoE, apoA-IV, lipoprotein lipase (LPL), ATP-binding cassette protein 1 (ABC1), lecithin cholesterol acyltransferase (LCAT), and scavenger receptor B-I (SR-BI). In contrast, some gene products are potential candidates for inhibition, including apoA-II, cholesteryl ester transfer protein (CETP), and hepatic lipase. The next decade will witness the transition from preclinical studies to clinical trials of a variety of new therapies targeted toward HDL metabolism and atherosclerosis.

Charting the fate of the "good cholesterol": identification and characterization of the high-density lipoprotein receptor SR-BI

Risk for cardiovascular disease due to atherosclerosis increases with increasing concentrations of low-density lipoprotein (LDL) cholesterol and is inversely proportional to the levels of high-density lipoprotein (HDL) cholesterol. The receptor-mediated control of plasma LDL levels has been well understood for over two decades and has been a focus for the pharmacologic treatment of hypercholesterolemia. In contrast, the first identification and characterization of a receptor that mediates cellular metabolism of HDL was only recently reported. This receptor, called scavenger receptor class B type I (SR-BI), is a fatty acylated glycoprotein that can cluster in caveolae-like domains on the surfaces of cultured cells. SR-BI mediates selective lipid uptake from HDL to cells. The mechanism of selective lipid uptake is fundamentally different from that of classic receptor-mediated endocytic uptake via coated pits and vesicles (e.g. the LDL receptor pathway) in that it involves efficient receptor-mediated transfer of the lipids, but not the outer shell proteins, from HDL to cells. In mice, SR-BI plays a key role in determining the levels of plasma HDL cholesterol and in mediating the regulated, selective delivery of HDL-cholesterol to steroidogenic tissues and the liver. Significant alterations in SR-BI expression can result in cardiovascular and reproductive disorders. SR-BI may play a similar role in humans; thus, modulation of its activity may provide the basis of future approaches to the treatment and prevention of atherosclerotic disease.

Dietary cholic acid lowers plasma levels of mouse and human apolipoprotein A-I primarily via a transcriptional mechanism

To induce dietary atherosclerosis in mice, high-fat/high-cholesterol (HF) diets are frequently supplemented with cholic acid (CA). This diet produces low plasma levels of high-density lipoprotein (HDL) and high levels of low-density lipoprotein (LDL). However, HF diets without any added CA, which more closely resemble human diets, increase levels of both HDL and LDL, suggesting that CA may be responsible for the lowering of HDL. Our aim was to examine the potential mechanism responsible for the lowering of HDL. Nontransgenic (NTg) C57BL mice and apoA-I-transgenic (apoAI-Tg) mice, with greatly increased basal apoA-I and HDL levels, were used. Mice were fed the following four diets: control (C), high-fat/high-cholesterol (HF), control and 1% cholate (CA) and HF + CA. Dietary CA reduced plasma HDL levels by 35% in NTg and 250% in apoAI-Tg mice, independent of the fat or cholesterol content of the diet. Hepatic apoA-I mRNA decreased 30% in NTg and 180% in apoAI-Tg mice. Hepatic apoA-I synthesis and apoA-I mRNA transcription rates also decreased in parallel with apoA-I mRNA levels, suggesting that the CA-induced decreases in plasma apoA-I levels occurred primarily via decreasing apoA-I mRNA transcription rates. An HF diet increased HDL levels 1.8-fold in NTg and 1.5-fold in apoAI-Tg mice. Addition of CA to the HF diet lowered HDL levels by 1.6-fold in NTg and 2. 5-fold in apoAI-Tg mice. Transfection studies with the apoA-I promoter suggested the presence of a putative cis-acting element responsible for the CA-mediated down-regulation of the apoA-I promoter activity. Measurements of apoA-I regulatory protein-1 (ARP-1) mRNA, a negative regulator of the apoA-I gene in the mouse liver showed that CA increased the ARP-1 mRNA levels. Because apoA-I gene transcription alone was not sufficient to account for the lowering of plasma HDL levels, scavenger receptor-B1 (SR-B1) and hepatic lipase (HL) mRNAs levels were quantitated. The levels of SR-B1 and HL mRNA were not changed by dietary CA. These studies suggest that dietary cholate regulates plasma levels of apoA-I primarily by a transcriptional mechanism via a putative bile acid response element involving a negative regulator of apoA-I, and partly by an unidentified post-transcriptional mechanism.

Hepatic lipase and HDL metabolism

Hepatic lipase is a lipolytic enzyme that has been suggested to have a role in HDL metabolism. Evidence suggests that HDL-cholesterol level is at least partly regulated by hepatic lipase level. Recent studies have shown that hepatic lipase not only hydrolyzes triglyceride and phospholipid in HDL, but also stimulates HDL cholesterol ester uptake by hepatocytes. Therefore, hepatic lipase, together with lipid transfer proteins, determines both HDL-cholesterol level and its function in reverse cholesterol transport. These conclusions are based on observations from in-vitro model substrate studies, cell culture studies, transgenic animal studies, and clinical studies. At present time, it is not known whether hepatic lipase action increases or decreases risk of developing atherosclerosis.

Endothelial lipase: a new member of the triglyceride lipase gene family

The triglyceride lipase gene family plays a central role in intestinal lipid absorption, energy homeostasis, lipoprotein metabolism, and atherosclerosis. A new member of this gene family, termed endothelial lipase, was recently reported. The presence of key functional motifs, the endothelial synthesis, the enzymatic profile, and the in-vivo metabolic effects of endothelial lipase suggest that, like other members of this gene family, endothelial lipase may play a role in energy delivery to tissues and in modulating lipoprotein metabolism, and could impact on atherogenesis.

Genes influencing HDL metabolism: new perspectives and implications for atherosclerosis prevention

Atherosclerotic cardiovascular disease (ASCVD) is the most common cause of morbidity and mortality in Western societies. Current therapies, such as reduction of plasma cholesterol, significantly reduce, but do not come close to eliminating, the complications of ASCVD. Therefore, novel therapeutic approaches to the prevention of acute coronary events and progression of atherosclerosis are still needed. The complex metabolism of high density lipoproteins represents an attractive potential target for therapeutic intervention. Here, we will discuss those components of the high density lipoprotein metabolism and lipid transport pathways that are potential preventative or therapeutic targets for ASCVD.

Intracellular activation of rat hepatic lipase requires transport to the Golgi compartment and is associated with a decrease in sedimentation velocity

Hepatic lipase (HL) is an N-glycoprotein that acquires triglyceridase activity somewhere during maturation and secretion. To determine where and how HL becomes activated, the effect of drugs that interfere with maturation and intracellular transport of HL protein was studied using freshly isolated rat hepatocytes. Carbonyl cyanide m-chlorophenyl hydrazone (CCCP), castanospermine, monensin, and colchicin all inhibited secretion of HL without affecting its specific enzyme activity. The specific enzyme activity of intracellular HL was decreased by 25-50% upon incubation with CCCP or castanospermine, and increased 2-fold with monensin and colchicin. Glucose trimming of HL protein was not affected by CCCP, as indicated by digestion of immunoprecipitates with jack bean alpha-mannosidase. Pulse labeling experiments with [(35)S]methionine indicated that conversion of the 53-kDa precursor to the 58-kDa form, nor the development of endoglycosidase H-resistance, were essential for acquisition of enzyme activity. In sucrose gradients, HL protein from secretion media sedimented as a homogeneous band of about 5.8 S, whereas HL protein from the cell lysates migrated as a broad band extending from 5.8 S to more than 8 S. With both sources, HL activity was exclusively associated with the 5.8 S HL protein form. We conclude that glucose trimming of HL protein in the endoplasmic reticulum is not sufficient for activation; full activation occurs during or after transport from the endoplasmic reticulum to the Golgi and is associated with a decrease in sedimentation velocity.

In vivo evidence for both lipolytic and nonlipolytic function of hepatic lipase in the metabolism of HDL

To investigate the in vivo role that hepatic lipase (HL) plays in HDL metabolism independently of its lipolytic function, recombinant adenovirus (rAdV) expressing native HL, catalytically inactive HL (HL-145G), and luciferase control was injected in HL-deficient mice. At day 4 after infusion of 2 x 10(8) plaque-forming units of rHL-AdV and rHL-145G-AdV, similar plasma concentrations were detected in postheparin plasma (HL=8.4+/-0.8 microg/mL and HL-145G=8.3+/-0.8 microg/mL). Mice expressing HL had significant reductions of cholesterol (-76%), phospholipids (PL; -68%), HDL cholesterol (-79%), apolipoprotein (apo) A-I (-45%), and apoA-II (-59%; P<0.05 for all), whereas mice expressing HL-145G decreased their cholesterol (-49%), PL (-40%), HDL cholesterol (-42%), and apoA-II (-89%; P<0.005 for all) but had no changes in apoA-I. The plasma kinetics of (125)I-labeled apoA-I HDL, (131)I-labeled apoA-II HDL, and [(3)H]cholesteryl ester (CE) HDL revealed that compared with mice expressing luciferase control (fractional catabolic rate [FCR] in d(-1): apoA-I HDL=1.3+/-0.1; apoA-II HDL=2.1+/-0; CE HDL=4.1+/-0.7), both HL and HL-145G enhanced the plasma clearance of CEs and apoA-II present in HDL (apoA-II HDL=5.6+/-0.5 and 4.4+/-0.2; CE HDL=9.3+/-0. 0 and 8.3+/-1.1, respectively), whereas the clearance of apoA-I HDL was enhanced in mice expressing HL (FCR=4.6+/-0.3) but not HL-145G (FCR=1.4+/-0.4). These combined findings demonstrate that both lipolytic and nonlipolytic functions of HL are important for HDL metabolism in vivo. Our study provides, for the first time, in vivo evidence for a role of HL in HDL metabolism independent of lipolysis and provides new insights into the role of HL in facilitating distinct metabolic pathways involved in the catabolism of apoA-I- versus apoA-II-containing HDL.

Transgenic animals and nutrition research

Transgenic animals are useful tools for the study of biological functions of proteins and secondary gene products synthesized by the action of protein catalysts. Research in nutrition and allied fields is benefiting from their use as models to contrast normal and altered metabolism. Although food, nutritional products, and ingredients from transgenic animals have not yet reached consumers, the technologies for their production are maturing and yielding exciting results in experimental and farm animals. Regulatory governmental bodies are already issuing guidelines and legislation in anticipation of the advent of these products and ingredients. This review summarizes available technology for the production of transgenic animals, discusses their scientific and commercial potential, and examines ancillary issues relevant to the field of nutrition.

Somatic gene therapy for dyslipidemias

Somatic gene transfer is a valuable tool for the in vivo evaluation of lipoprotein metabolism. It has been used to dissect metabolic pathways, to establish structure-function relationships of various gene products, and to evaluate conventional lipid-lowering and novel therapeutic genes for the treatment of lipoprotein disorders. In this article we review some general aspects of somatic gene therapy and the different vehicles used for the delivery of therapeutic genes. We highlight some recent advances in adenoviral vector development that make this vector an attractive system for clinical trials.

Atorvastatin increases low-density lipoprotein size and enhances high-density lipoprotein cholesterol concentration in male, but not in female patients with familial hypercholesterolemia

The effects of atorvastatin (Lipitor) were evaluated in 40 patients with familial hypercholesterolemia (FH). Following a 6 week drug-free baseline period 20 male and 20 female patients were treated with atorvastatin 40 mg once daily (QD) for the initial 6 weeks increasing to 80 mg QD during the following 6 weeks. Atorvastatin 40 and 80 mg resulted in a dose related reduction in LDL cholesterol of 44 and 50% (P<0.001), respectively. The reduction of triglycerides (TG) was 35% (P<0.001) with 40 and 80 mg atorvastatin. The lipoprotein lipase and the hepatic lipase activity decreased dose independently by 13% (P<0.05) and 18% (P<0.01), respectively. In males, a dose independent increase in high-density lipoprotein (HDL) cholesterol concentration was observed of 8%, (P<0.05). In females, the HDL cholesterol concentration did not change. Baseline LDL size in the females was significantly larger than in the males, being 268+/-6 A and 264+/-8 A (P<0.05), respectively. In males LDL size increased significantly from 264+/-8 A at baseline to269+/-6 A at 40 mg (P<0.05) and to 270+/-5 A (P<0.05) at 80 mg atorvastatin. In females LDL size did not change upon treatment with atorvastatin 40 and 80 mg QD. In conclusion, atorvastatin has the ability to decrease cholesterol and triglyceride concentrations as well as the activity of both lipoprotein and hepatic lipase activity. Additionally it has a favorable effect on LDL size and HDL cholesterol concentration in male, but not in female FH patients.

HDL metabolism in hypertriglyceridemic states: an overview

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

Remodelling of high density lipoproteins by plasma factors

Evidence that the high density lipoproteins (HDL) in human plasma are antiatherogenic has stimulated considerable interest in the factors which regulate their structure and function. Plasma HDL consist of a number of subpopulations of particles of varying size, density and composition. This structural heterogeneity is caused by the continual remodelling of individual HDL subpopulations by various plasma factors. One of the consequences of this remodelling is that the HDL subpopulations in plasma are functionally diverse, particularly in terms of their antiatherogenic properties. This review documents what is currently known about the interaction of HDL with plasma factors and presents an overview of the remodelling of HDL which occurs as a consequence of those interactions.

Atheroprotective mechanisms of HDL

The aim of this review was to bring together results obtained from studies on different aspects of HDL as related to CHD and atherosclerosis. As atherosclerosis is a multistep process, the various components of HDL can intervene at different stages, such as induction of monocyte adhesion molecules, prevention of LDL modification and removal of excess cholesterol by reverse cholesterol transport. Transgenic technology has provided a model for atherosclerosis, and permitted evaluation of the contributions of different HDL components towards the global effect. The availability of apo AIV transgenic mice amplified the results obtained from apo AI overexpressors with respect to prevention of atherosclerosis. Prevention of atherosclerosis in apo E deficient mice by relatively small amounts of macrophage derived apo E may open new possibilities for therapeutic intervention. Contrary to early notions, increased plasma levels of CETP, even in the presence of low but functionally normal HDL, were atheroprotective. The extent to which paraoxonase and apo J participate in prevention of human atherosclerosis needs further evaluation. The findings that LCAT overexpression in rabbits was atheroprotective in contrast to increase in atherosclerosis in h LCAT tg mice, which was only partially corrected by CETP expression, call for some caution in the extrapolation of results from transgenic animals to humans. The important discovery of SR-BI as the receptor for selective uptake of CE from HDL revived interest in the clearance of CE from plasma. This pathway supplies also the vital precursor for steroidogenesis in adrenals and gonads and was shown to be dependent on apo AI.

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

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

A novel endothelial-derived lipase that modulates HDL metabolism

High-density lipoprotein (HDL) cholesterol levels are inversely associated with risk of atherosclerotic cardiovascular disease. At least 50% of the variation in HDL cholesterol levels is genetically determined, but the genes responsible for variation in HDL levels have not been fully elucidated. Lipoprotein lipase (LPL) and hepatic lipase (HL), two members of the triacylglyerol (TG) lipase family, both influence HDL metabolism and the HL (LIPC) locus has been associated with variation in HDL cholesterol levels in humans. We describe here the cloning and in vivo functional analysis of a new member of the TG lipase family. In contrast to other family members, this new lipase is synthesized by endothelial cells in vitro and thus has been termed endothelial lipase (encoded by the LIPG gene). EL is expressed in vivo in organs including liver, lung, kidney and placenta, but not in skeletal muscle. In contrast to LPL and HL, EL has a lid of only 19 residues. EL has substantial phospholipase activity, but less triglyceride lipase activity. Overexpression of EL in mice reduced plasma concentrations of HDL cholesterol and its major protein apolipoprotein A-I. The endothelial expression, enzymatic profile and in vivo effects of EL suggest that it may have a role in lipoprotein metabolism and vascular biology.

Hepatic lipase

Hepatic lipase (HL) is an important enzyme that is involved in the metabolism of chylomicrons, intermediate density lipoproteins, and high density lipoproteins. HL may affect the liver uptake of remnant lipoproteins by modifying their compositions. HL also participates in the reverse cholesterol transport, thereby influencing the process of atherosclerosis. Several new functions of HL have recently been revealed. In this article, we review some of the recent progress based on studies using transgenic animals, with an emphasis on HL functions in remnant metabolism and atherosclerosis.

Endotoxin and interleukin-1 decrease hepatic lipase mRNA levels

The acute phase response induces a multitude of changes in lipoprotein metabolism including hypertriglyceridemia, triglyceride enriched LDL, and decreased HDL levels accompanied by changes in HDL composition including increased free cholesterol and triglycerides and a decrease in esterified cholesterol. Here we demonstrate that endotoxin (LPS) induces a 56% decrease in hepatic lipase activity in liver and a 45% decrease in hepatic lipase activity in post heparin plasma in Syrian hamsters. LPS treatment also produces a marked decrease in hepatic lipase mRNA levels in the liver. Half maximal reduction in hepatic lipase mRNA levels occurred at approximately 0.2 microg LPS/100 g BW with a maximal decrease at 1.0 microg/100 g BW ( > 90% decrease), indicating that inhibition of hepatic lipase is a sensitive host response to LPS. Additionally, IL-1 produced a marked decrease in hepatic lipase mRNA levels while TNF had no effect. Moreover, IL-1 treatment of HepG2 cells in vitro also decreased hepatic lipase mRNA levels suggesting that IL-1 can directly regulate hepatic lipase expression in liver cells. LPS decreased hepatic lipase mRNA levels in control as well as IL-1 type 1 receptor deficient mice indicating that IL-1 action is not absolutely essential and that several cytokines and/or small molecular mediators can regulate hepatic lipase during the acute phase response. The LPS and IL-1 induced decrease in hepatic lipase could have several consequences including decreasing the clearance of triglyceride rich lipoprotein particles and producing an increase in triglyceride rich HDL. The decrease in hepatic lipase activity and mRNA levels may be part of a series of coordinated changes in lipoprotein metabolism that occur during the acute phase response. These changes may be initially beneficial to the host but if present for an extended period may be proatherogenic.

Common variants in the promoter of the hepatic lipase gene are associated with lower levels of hepatic lipase activity, buoyant LDL, and higher HDL2 cholesterol

Increased hepatic lipase (HL) activity is associated with small, dense, low density lipoprotein (LDL) and low high density lipoprotein2 (HDL2) cholesterol (-C) levels. A polymorphism in the promoter region of the HL gene (LIPC) is associated with HDL-C levels. To test whether this association is mediated by differences in HL activity between different LIPC promoter genotypes, the LIPC promoter polymorphism at position -250 (G-->A), HL activity, LDL buoyancy, and HDL-C levels were studied in white normolipidemic men and men with coronary artery disease (CAD). The less common A allele (frequency=0.21 and 0.25 in normal and CAD subjects, respectively) was associated with lower HL activity (P<0.005 by ANOVA) and buoyant LDL particles (P</=0.01) in both groups. Normal and CAD subjects heterozygous for the A allele had lower HL activity (by 24% and 29%, respectively) and significantly more buoyant LDL particles. Homozygosity for this allele (AA) was associated with an even lower HL activity in normal (-26%) and CAD (-46%) subjects. The A allele was associated with higher HDL2-C in CAD patients (P=0.007); heterozygotes and homozygotes for the A allele had a 92% and a 140% higher HDL2-C level (P<0.01) than did GG individuals. In a small number of normolipidemic subjects, the same trend in HDL2-C was seen. In a univariate analysis, the LIPC genotype accounted for 20% to 32% of the variance in HL levels among normal subjects and CAD patients, respectively. After adjustment for HL, the association between LIPC genotype and LDL buoyancy was no longer significant, suggesting that the effect of LIPC genotype on LDL buoyancy is mediated by its effects on HL activity. The LIPC A allele was more frequent in Japanese-Americans and African-Americans than in whites. In summary, these results suggest that variants in the LIPC promoter may significantly contribute to the variance in levels of HL activity and consequently, to the prevalence of the atherogenic small, dense, LDL particles and low HDL2-C levels.

Lipoproteins and atherogenesis

Like many complex disease processes, atherogenesis represents the interaction of an array of genetic and environmental factors. From nonhuman animal models to the investigation of epidemiologic factors in man, no single, overriding cause for the development of this indolent vascular disease has been identified. However, the cholesterol-enriched lipoprotein particles are closely tied to the development of the disease. The genetic and environmental influences on the concentrations of specific lipoprotein subspecies provide a context for identifying patients at risk as well as for developing effective therapeutic strategies to influence and prevent the sequelae of atherogenesis.

Effect of thyroid dysfunction on high-density lipoprotein subfraction metabolism: roles of hepatic lipase and cholesteryl ester transfer protein

To investigate the effect of thyroid dysfunction on high-density lipoprotein (HDL) metabolism, we measured HDL subfractions, apolipoprotein A-I containing particles (LpA-I and LpA-I:A-II), and the activities of enzymes involved in the remodeling and metabolism of HDL [namely hepatic lipase (HL), lipoprotein lipase, and cholesteryl ester transfer protein (CETP)] in 18 hyperthyroid and 17 hypothyroid patients before and after treatment. HDL was subfractionated by density gradient ultracentrifugation, and LpA-I was analyzed by electroimmunodiffusion. The major changes were found in the HDL2 subfraction and in LpA-I particles. HDL2-C and LpA-I were reduced in hyperthyroidism (P < 0.01, P < 0.05, respectively) and increased in hypothyroidism (both P < 0.05) compared with their respective euthyroid matched controls. Changes in HDL2-cholesterol were reversed after treatment in both hyper- and hypothyroid patients, and LpA-I also decreased in the hypothyroid patients after treatment. HL (P < 0.05) and CETP activities (P < 0.05) were elevated in hyperthyroidism and reduced in hypothyroidism (P < 0.05, P < 0.01 respectively) and both were related to free T4 levels. The changes in HDL2-C and LpA-I correlated significantly with changes in HL after treatment but not with CETP or lipoprotein lipase. In summary, HDL metabolism was altered in thyroid dysfunction, and the effect of thyroid hormone on HDL was mediated mainly via its effect on HL activity.

Hepatic lipase affects both HDL and ApoB-containing lipoprotein levels in the mouse

Transgenic mice were created overproducing a range of human HL (hHL) activities (4-23-fold increase) to further examine the role of hepatic lipase (HL) in lipoprotein metabolism. A 5-fold increase in heparin releasable HL activity was accompanied by moderate (approx. 20%) decreases in plasma total and high density lipoprotein (HDL) cholesterol and phospholipid (PL) but no significant change in triglyceride (TG). A 23-fold increase in HL activity caused a more significant decrease in plasma total and HDL cholesterol, PL and TG (77%, 64%, 60%, and 24% respectively), and a substantial decrease in lipoprotein lipids amongst IDL, LDL and HDL fractions. High levels of HL activity diminished the plasma concentration of apoA-I, A-II and apoE (76%, 48% and 75%, respectively). In contrast, the levels of apoA-IV-containing lipoproteins appear relatively resistant to increased titers of hHL activity. Increased hHL activity was associated with a progressive decrease in the levels and an increase in the density of LpAI and LpB48 particles. The increased rate of disappearance of 125I-labeled human HDL from the plasma of hHL transgenic mice suggests increased clearance of HDL apoproteins in the transgenic mice. The effect of increased HL activity on apoB100-containing lipoproteins was more complex. HL-deficient mice have substantially decreased apoB100-containing low density lipoproteins (LDL) compared to controls. Increased HL activity is associated with a transformation of the lipoprotein density profile from predominantly buoyant (VLDL/IDL) lipoproteins to more dense (LDL) fractions. Increased HL activity from moderate (4-fold) to higher (5-fold) levels decreased the levels of apoB100-containing particles. Thus, at normal to moderately high levels in the mouse, HL promotes the metabolism of both HDL and apoB-containing lipoproteins and thereby acts as a key determinant of plasma levels of both HDL and LDL.

The role of hepatic lipase in lipoprotein metabolism and atherosclerosis

In addition to its traditional role in the hydrolysis of lipoprotein triglycerides and phospholipids, recent studies have implicated hepatic lipase in other aspects of cellular lipid and/or lipoprotein metabolism and atherosclerosis. Hepatic lipase may serve as a ligand that mediates the interaction of lipoproteins to cell surface receptors and/or proteoglycans as well as modulating aortic lesion development in different animal models. Over the past several years significant advances have been made in our understanding of new, alternative mechanisms by which hepatic lipase may modulate lipoprotein metabolism and the development of atherosclerosis in vivo.

Effects of testosterone replacement on HDL subfractions and apolipoprotein A-I containing lipoproteins

OBJECTIVES

Gonadal steroids are important regulators of lipoprotein metabolism. The aims of this study were to determine the effects of a minimum effective dose of testosterone replacement on high density lipoprotein (HDL) subfractions and apolipoprotein (apo) A-I containing particles (lipoprotein (Lp)A-I) and LpA-I:A-II) in hypogonadal men with primary testicular failure and to investigate the underlying mechanisms of these changes.

MEASUREMENTS

Eleven Chinese hypogonadal men were started on testosterone enanthate 250 mg intramuscularly at 4-weekly intervals. HDL was subfractionated by density gradient ultracentrifugation and LpA-I was analysed by electro-immunodiffusion after 3, 6 and 12 weeks of treatment. Plasma cholesteryl ester transfer protein (CETP) activity and lipolytic enzymes activities in post-heparin plasma were measured to determine the mechanisms underlying testosterone-induced changes in HDL.

RESULTS

The dosage of testosterone enanthate used in the present study resulted in suboptimal trough testosterone levels. No changes were seen in plasma total cholesterol, triglyceride, low density lipoprotein cholesterol (LDL-C,) apo B and apo(a) after 12 weeks. There was a drop in HDL3-C compared to baseline (0.82 +/- 0.17 mmol/l vs. 0.93 +/- 0.13, P < 0.01) whereas a small but significant increase was seen in HDL2-C (0.21 +/- 0.13 mmol/l vs. 0.11 +/- 0.09, P < 0.05). Plasma apo A-I decreased after treatment (1.34 +/- 0.25 g/l vs. 1.50 +/- 0.29, P < 0.01), due to a reduction in LpA-I:A-II particles (0.86 +/- 0.18 g/l vs. 0.99 +/- 0.24, P < 0.01). No changes were observed in the levels of LpA-I particles. No significant changes were seen in plasma CETP and lipoprotein lipase activities after testosterone replacement but there was a transient increase in hepatic lipase (HL) activity at weeks 3 and 6. The decrease in HDL correlated with the increase in HL activity (r = 0.62, P < 0.05).

CONCLUSIONS

Testosterone replacement in the form of parenteral testosterone ester given 4-weekly, although unphysiological, was not associated with unfavourable changes in lipid profiles. The reduction in HDL was mainly in HDL3-C and in LpA-I:A-II particles and not in the more anti-atherogenic HDL2 and LpA-I particles. The changes in HDL subclasses were mainly mediated through the effect of testosterone on hepatic lipase activity.

Overexpression of hepatic lipase in transgenic mice decreases apolipoprotein B-containing and high density lipoproteins. Evidence that hepatic lipase acts as a ligand for lipoprotein uptake

To determine the mechanisms by which human hepatic lipase (HL) contributes to the metabolism of apolipoprotein (apo) B-containing lipoproteins and high density lipoproteins (HDL) in vivo, we developed and characterized HL transgenic mice. HL was localized by immunohistochemistry to the liver and to the adrenal cortex. In hemizygous (hHLTg+/0) and homozygous (hHLTg+/+) mice, postheparin plasma HL activity increased by 25- and 50-fold and plasma cholesterol levels decreased by 80% and 85%, respectively. In mice fed a high fat, high cholesterol diet to increase endogenous apoB-containing lipoproteins, plasma cholesterol decreased 33% (hHLTg+/0) and 75% (hHLTg+/+). Both apoB-containing remnant lipoproteins and HDL were reduced. To extend this observation, the HL transgene was expressed in human apoB transgenic (huBTg) and apoE-deficient (apoE-/-) mice, both of which have high plasma levels of apoB-containing lipoproteins. (Note that the huBTg mice that were used in these studies were all hemizygous for the human apoB gene.) In both the huBTg,hHLTg+/0 mice and the apoE-/-,hHLTg+/0 mice, plasma cholesterol decreased by 50%. This decrease was reflected in both the apoB-containing and the HDL fractions. To determine if HL catalytic activity is required for these decreases, we expressed catalytically inactive HL (HL-CAT) in apoE-/- mice. The postheparin plasma HL activities were similar in the apoE-/- and the apoE-/-,HL-CAT+/0 mice, reflecting the activity of the endogenous mouse HL and confirming that the HL-CAT was catalytically inactive. However, the postheparin plasma HL activity was 20-fold higher in the apoE-/-,hHLTg+/0 mice, indicating expression of the active human HL. Immunoblotting demonstrated high levels of human HL in postheparin plasma of both apoE-/-,hHLTg+/0 and apoE-/-,HL-CAT+/0 mice. Plasma cholesterol and apoB-containing lipoprotein levels were approximately 60% lower in apoE-/-,HL-CAT+/0 mice than in apoE-/- mice. However, the HDL were only minimally reduced. Thus, the catalytic activity of HL is critical for its effects on HDL but not for its effects on apoB-containing lipoproteins. These results provide evidence that HL can act as a ligand to remove apoB-containing lipoproteins from plasma.

Heparan sulfate proteoglycans participate in hepatic lipaseand apolipoprotein E-mediated binding and uptake of plasma lipoproteins, including high density lipoproteins

High density lipoprotein (HDL) particles and HDL cholesteryl esters are taken up by both receptor-mediated and non-receptor-mediated pathways. Here we show that cell surface heparan sulfate proteoglycans (HSPG) participate in hepatic lipase (HL)- and apolipoprotein (apo) E-mediated binding and uptake of mouse and human HDL by cultured hepatocytes. The HL secreted by HL-transfected McA-RH7777 cells enhanced both HDL binding at 4 degrees C (approximately 2-4-fold) and HDL uptake at 37 degrees C (approximately 2-5-fold). The enhanced binding and uptake of HDL were partially inhibited by the 39-kDa protein, an inhibitor of low density lipoprotein receptor-related protein (LRP), but were almost totally blocked by heparinase, which removes the sulfated glycosaminoglycan chains from HSPG. Therefore, HL may mediate the uptake of HDL by two pathways: an HSPG-dependent LRP pathway and an HSPG-dependent but LRP-independent pathway. The HL-mediated binding and uptake of HDL were only minimally reduced when catalytically inactive HL or LRP binding-defective HL was substituted for wild-type HL, indicating that much of the HDL uptake required neither HL binding to the LRP nor lipolytic processing. To study the role of HL in facilitating the selective uptake of cholesteryl esters, we used HDL into which radiolabeled cholesteryl ether had been incorporated. HL increased the selective uptake of HDL cholesteryl ether; this enhanced uptake was reduced by more than 80% by heparinase but was unaffected by the 39-kDa protein. Like HL, apoE enhanced the binding and uptake of HDL (approximately 2-fold) but had little effect on the selective uptake of HDL cholesteryl ether. In the presence of HL, apoE did not further increase the uptake of HDL, and at a high concentration apoE impaired or decreased the HL-mediated uptake of HDL. Therefore, HL and apoE may utilize similar (but not identical) binding sites to mediate HDL uptake. Although the relative importance of cell surface HSPG in the overall metabolism of HDL in vivo remains to be determined, cultured hepatocytes clearly displayed an HSPG-dependent pathway that mediates the binding and uptake of HDL. This study also demonstrates the importance of HL in enhancing the binding and uptake of remnant and low density lipoproteins via an HSPG-dependent pathway.

Reverse cholesterol transport--a review of the process and its clinical implications

OBJECTIVES

This review article will summarize the current knowledge surrounding the reverse cholesterol transport system; the process, the effect of mutations in genes coding for proteins which function in the system, and the possible clinical implications of these alterations.

RESULTS

High-density lipoprotein-cholesterol (HDL-C) concentration is a marker for the reverse cholesterol transport (RCT) system, whereby cholesterol is returned from peripheral cells to the liver for reuse or excretion in the bile. Increased HDL-C concentrations are generally accepted to be protective against the future development of atherosclerosis and coronary artery disease (CAD), but recent evidence has indicated that the underlying cause of the increased HDL-C may affect whether it is protective or detrimental. The major steps in the RCT pathway are the efflux of free cholesterol from cells and binding by pre-beta HDL, esterification of HDL-bound cholesterol by lecithin cholesterol acyl transferase (LCAT), cholesteryl ester transfer protein (CETP) mediated exchange of cholesteryl ester and triglycerides between HDL and apo B-containing particles, and hepatic lipase (HL) mediated uptake of cholesterol and triglycerides by the liver. Mutations in proteins active in the RCT pathway can shed light on the functions and control of the various steps in the system. LCAT deficiency, leading to greatly reduced HDL and fish eye disease, is not usually associated with increased risk of CAD. Several new mutations in LCAT have recently been reported, however, which do result in CAD. Mutations leading to reduced CETP activity result in less CE being directed into apo-B containing particles and more remaining in the HDL. This has been associated with increased HDL-C concentrations. The generally accepted hypothesis that reduced CETP activity leads to reduced CAD risk has been challenged by a number of recent publications, and has become an area of active investigation. Mutations leading to reduced HL activity are rare occurrences. To date, all have been associated with increased HDL-C concentrations and CAD.

CONCLUSION

The development of techniques to identify and characterize the functional significance of mutations in proteins involved in RCT will aid in the understanding of the mechanisms and control of this pathway.

Hepatic lipase deficiency increases plasma cholesterol but reduces susceptibility to atherosclerosis in apolipoprotein E-deficient mice

The effect of hepatic lipase (HL) deficiency on the susceptibility to atherosclerosis was tested using mice with combined deficiencies in HL and apoE. Mice lacking both HL and apoE (hhee) have a plasma total cholesterol of 917 +/- 252 mg/dl (n = 24), which is 184% that of mice lacking only apoE (HHee; 497 +/- 161 mg/dl, n = 20, p < 0. 001). The increase in cholesterol was mainly in beta-migrating very low density lipoproteins, although high density lipoprotein cholesterol (HDLc) was also increased (53 +/- 37 versus 20 +/- 13 mg/dl, p < 0.01). Despite the increase in plasma cholesterol, we found that HL deficiency significantly decreased aortic plaque sizes in female mice fed normal chow (31 x 10(3) +/- 22 x 10(3) microm2 in hhee versus 115 x 10(3) +/- 69 x 10(3) microm2 in HHee, p < 0.001). Reduction of plaque sizes was also observed in female heterozygous apoE-deficient mice fed an atherogenic diet (2 x 10(3) +/- 2.5 x 10(3) microm2 in hhEe versus 56 x 10(3) +/- 49 x 10(3) microm2 in HHEe, p < 0.01). Changes in aortic lesion size were not apparent in the small number of male mice studied. In HHee females, both HDLc and the capacity of high density lipoprotein (HDL) particles to promote cholesterol efflux from cultured cells were 26% of the wild type. The absence of HL in hhee females partially restored HDLc levels to 57% and cholesterol efflux to 55% of the wild type. Circulating pre-beta1-migrating HDL were present in all mutants, suggesting that there are alternative pathways in the formation of these pre-beta-HDL not involving apoE, HL, or cholesteryl ester transfer protein. The improved capacity to promote cholesterol efflux, together with increased HDL, may explain why these animals can overcome the increase in atherogenic lipoproteins.

Lipoprotein lipase correlates positively and hepatic lipase inversely with calcific atherosclerosis in homozygous familial hypercholesterolemia

Homozygous familial hypercholesterolemia (FH) is a rare genetic disorder that leads to premature atherosclerosis due to a defective LDL receptor. There is, however, a large degree of phenotypic heterogeneity at the level of atherosclerosis even in patients with identical mutations of the LDL receptor protein. Lipoprotein lipase (LPL) and hepatic lipase (HL) are crucial enzymes in lipoprotein metabolism, and both have been proposed as having proatherogenic as well as antiatherogenic effects. To evaluate a potential role for these enzymes in the severity of atherosclerosis, we correlated postheparin LPL mass and activity as well as HL activity with the volume of total calcific atherosclerosis (heart and thoracic aorta), coronary artery calcific atherosclerosis, and Achilles tendon width as measured by computed tomography in 15 FH homozygotes. LPL dimer and total mass were positively correlated with all three parameters (r = .65 to .87, P < .01) as was LPL activity (r = .52 to .63, P < .05). HL activity was negatively correlated with total and coronary artery calcified lesion volume (r = -.55 to .57, P < .05). In a multiple regression model of the coronary artery lesion volume, LPL dimer mass and HL activity together accounted for 84% of the variability (r = .92, P < .0001). In a multiple regression model of the total calcified lesion volume, HL activity, total cholesterol, age, and LPL dimer mass together accounted for 85% of the variability (r = .92, P = .0005). These data demonstrate a significant correlation of LPL mass and activity with the extent of calcific atherosclerosis in homozygous FH. It is not clear whether LPL is the cause or consequence of the observed correlation, but if the association between LPL and coronary artery lesions is also present in patients with other genetic dyslipoproteinemias, LPL could constitute a new risk factor for cardiovascular disease.

Transcription of the human hepatic lipase gene is modulated by multiple negative elements in HepG2 cells

The expression of the hepatic lipase (HL) gene is highly tissue specific. In order to identify cis-acting elements which regulate the expression of this gene in the liver, multiple deletion mutants of the 5'-flanking region of the HL gene fused to the human growth hormone gene were transfected in HepG2 cells, which normally produce HL. Transient expression assays indicated the presence of negative (at nucleotides (nt) -1576(/)-1342 and -623(/)-407) and positive (at nt -1862(/)-1576 and -50(/)-9) regulatory elements. Transfection of HeLa cells, which do not produce HL, with the same deletion constructs resulted in a similar pattern of promoter activities. However, additional negative (nt -138/-50) and positive (nt -407(/)-138) elements were found. DNase I footprint analysis of the proximal and distal HLpromoter sequences with HepG2 and HeLa cell nuclear extracts identified seven protected regions: A, nt -1540(/)-1527; B, -1505(/)-1473; C, -1467(/)-1460; D, -592(/)-577; E, -565(/)-545; F, -234(/)-220; and G, -70(/) -48. Sites A, B, C, D and E were located within regions containing negative regulatory elements. In order to determine which nuclear factor interacts with the negative elements, sites B, D and E were mutated and the effects of mutation on competition in a gel retardation assay and on promoter activity were studied. When the binding motif for AP1 in sites B, D and E was mutated, the specific DNA-protein complexes were not competed with the mutant oligonucleotides and promoter activity increased twofold. The magnitude of the increase is less than expected from the deletion analysis, and simultaneous mutations did not cause further increase in promoter activity, which suggests that other sites are involved in this negative modulation. These results suggest that the transcription of the HLgene in HepG2 cells is negatively modulated by multiple cis-acting negative elements and AP1-like nuclear factor may play some role in this modulation.

Human hepatic lipase subunit structure determination

Chinese hamster ovary cells were stably transfected with a human hepatic lipase (HL) cDNA. The recombinant enzyme was purified from culture medium in milligram quantities and shown to have a molecular weight, specific activity, and heparin affinity equivalent to HL present in human post-heparin plasma. The techniques of intensity light scattering, sedimentation equilibrium, and radiation inactivation were employed to assess the subunit structure of HL. For intensity light scattering, purified enzyme was subjected to size exclusion chromatography coupled to three detectors in series: an ultraviolet absorbance monitor, a differential refractometer, and a light scattering photometer. The polypeptide molecular weight (without carbohydrate contributions) was calculated using the measurements from the three detectors combined with the extinction coefficient of human HL. A single protein peak containing HL activity was identified and calculated to have a molecular mass of 107,000 in excellent agreement with the expected value for a dimer of HL (106.8 kDa). In addition, sedimentation equilibrium studies revealed that HL had a molecular mass (with carbohydrate contributions) of 121 kDa. Finally, to determine the smallest structural unit required for lipolytic activity, HL was subjected to radiation inactivation. Purified HL was exposed to various doses of high energy electrons at -135 degrees C; lipase activity decreased as a single exponential function of the radiation dose to less than 0.01% remaining activity. The target size of functional HL was calculated to be 109 kDa, whereas the size of the structural unit was determined to be 63 kDa. These data indicate that two HL monomer subunits are required for lipolytic activity, consistent with an HL homodimer. A model for active dimeric hepatic lipase is presented with implications for physiological function.