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

Identification of a Gain-of-Function LIPC Variant as a Novel Cause of Familial Combined Hypocholesterolemia

BACKGROUND

Atherosclerotic cardiovascular disease is the main cause of mortality worldwide and is strongly influenced by circulating low-density lipoprotein (LDL) cholesterol levels. Only a few genes causally related to plasma LDL cholesterol levels have been identified so far, and only 1 gene, ANGPTL3, has been causally related to combined hypocholesterolemia. Here, our aim was to elucidate the genetic origin of an unexplained combined hypocholesterolemia inherited in 4 generations of a French family.

METHODS

Using next-generation sequencing, we identified a novel dominant rare variant in the LIPC gene, encoding for hepatic lipase, which cosegregates with the phenotype. We characterized the impact of this LIPC-E97G variant on circulating lipid and lipoprotein levels in family members using nuclear magnetic resonance-based lipoprotein profiling and lipidomics. To uncover the mechanisms underlying the combined hypocholesterolemia, we used protein homology modeling, measured triglyceride lipase and phospholipase activities in cell culture, and studied the phenotype of APOE*3.Leiden.CETP mice after LIPC-E97G overexpression.

RESULTS

Family members carrying the LIPC-E97G variant had very low circulating levels of LDL cholesterol and high-density lipoprotein cholesterol, LDL particle numbers, and phospholipids. The lysophospholipids/phospholipids ratio was increased in plasma of LIPC-E97G carriers, suggestive of an increased lipolytic activity on phospholipids. In vitro and in vivo studies confirmed that the LIPC-E97G variant specifically increases the phospholipase activity of hepatic lipase through modification of an evolutionarily conserved motif that determines substrate access to the hepatic lipase catalytic site. Mice overexpressing human LIPC-E97G recapitulated the combined hypocholesterolemic phenotype of the family and demonstrated that the increased phospholipase activity promotes catabolism of triglyceride-rich lipoproteins by different extrahepatic tissues but not the liver.

CONCLUSIONS

We identified and characterized a novel rare variant in the LIPC gene in a family who presents with dominant familial combined hypocholesterolemia. This gain-of-function variant makes LIPC the second identified gene, after ANGPTL3, causally involved in familial combined hypocholesterolemia. Our mechanistic data highlight the critical role of hepatic lipase phospholipase activity in LDL cholesterol homeostasis and suggest a new LDL clearance mechanism.

The role of hepatic Surf4 in lipoprotein metabolism and the development of atherosclerosis in apoE-/- mice

Elevated plasma levels of low-density lipoprotein-C (LDL-C) increase the risk of atherosclerotic cardiovascular disease. Circulating LDL is derived from very low-density lipoprotein (VLDL) metabolism and cleared by LDL receptor (LDLR). We have previously demonstrated that cargo receptor Surfeit 4 (Surf4) mediates VLDL secretion. Inhibition of hepatic Surf4 impairs VLDL secretion, significantly reduces plasma LDL-C levels, and markedly mitigates the development of atherosclerosis in LDLR knockout (Ldlr^-/-) mice. Here, we investigated the role of Surf4 in lipoprotein metabolism and the development of atherosclerosis in another commonly used mouse model of atherosclerosis, apolipoprotein E knockout (apoE^-/-) mice. Adeno-associated viral shRNA was used to silence Surf4 expression mainly in the liver of apoE^-/- mice. In apoE^-/- mice fed a regular chow diet, knockdown of Surf4 expression significantly reduced triglyceride secretion and plasma levels of non-HDL cholesterol and triglycerides without causing hepatic lipid accumulation or liver damage. When Surf4 was knocked down in apoE^-/- mice fed the Western-type diet, we observed a significant reduction in plasma levels of non-HDL cholesterol, but not triglycerides. Knockdown of Surf4 did not increase hepatic cholesterol and triglyceride levels or cause liver damage, but significantly diminished atherosclerosis lesions. Therefore, our findings indicate the potential of hepatic Surf4 inhibition as a novel therapeutic strategy to reduce the risk of atherosclerotic cardiovascular disease.

The Lipid-Modulating Effect of Tangeretin on the Inhibition of Angiopoietin-like 3 (ANGPTL3) Gene Expression through Regulation of LXRα Activation in Hepatic Cells

The excessive accumulation of TG-rich lipoproteins (TGRLs) in plasma is associated with dyslipidemia and atherosclerotic cardiovascular diseases (ASCVDs). Tangeretin is a bioactive pentamethoxyflavone mainly found in citrus peels, and it has been reported to protect against hyperlipidemia, diabetes, and obesity. The aim of this study was to investigate the lipid-modulating effects and the underlying mechanisms of tangeretin action in hepatic cells. Transcriptome and bioinformatics analyses with the Gene Ontology (GO) database showed that tangeretin significantly regulated a set of 13 differentially expressed genes (DEGs) associated with the regulation of lipoprotein lipase (LPL) activity. Among these DEGs, angiopoietin-like 3 (ANGPTL3), an essential inhibitor of LPL catalytic activity that regulates TGRL metabolism in plasma, was markedly downregulated by tangeretin. We demonstrated that tangeretin significantly inhibited the mRNA expression of ANGPTL3 in HepG2 and Huh-7 cells. Tangeretin treatment of hepatic cells also reduced the levels of both intracellular and secreted ANGPTL3 proteins. Moreover, we found that inhibition of ANGPTL3 production by tangeretin augmented LPL activity. We further demonstrated that the transcriptional activity of the ANGPTL3 promoter was significantly attenuated by tangeretin, and we identified a DNA element located between the −250 and −121 positions that responded to tangeretin. Furthermore, we found that tangeretin did not alter the levels of the nuclear liver X receptor α (LXRα) protein, an essential transcription factor that binds to the tangeretin-responsive element, but it can counteract LXRα-mediated ANGPTL3 transcription. On the basis of molecular docking analysis, tangeretin was predicted to bind to the ligand-binding domain of LXRα, which would result in suppression of LXRα activation. Our findings support the hypothesis that tangeretin exerts a lipid-lowering effect by modulating the LXRα-ANGPTL3-LPL pathway, and thus, it can be used as a potential phytochemical for the prevention or treatment of dyslipidemia.

Dietary squalene modifies plasma lipoproteins and hepatic cholesterol metabolism in rabbits

To evaluate the effects of squalene, the main unsaponifiable component of virgin olive oil, on lipid metabolism, two groups of male New Zealand rabbits were fed a 1% sunflower oil-enriched regular diet or the same diet containing 0.5% squalene for 4 weeks. Plasma triglycerides, total- and HDL-cholesterol and their lipoproteins were assayed. Analyses of hepatic lipid droplets, triglycerides, total- and non-esterified cholesterol, squalene, protein and gene expression, and cholesterol precursors were carried out. In the jejunum, the squalene content and mRNA and protein APOB expressions were measured. Finally, we studied the effect of cholesterol precursors in AML12 cells. Squalene administration significantly increased plasma total cholesterol, mainly carried as non-esterified cholesterol in IDL and large LDL, and corresponded to an increased number of APOB100-containing particles without accumulation of triglycerides and decreased reactive oxygen species. Despite no significant changes in the APOB content in the jejunum, the latter displayed increased APOB mRNA and squalene levels. Increases in the amounts of non-esterified cholesterol, squalene, lanosterol, dihydrolanosterol, lathosterol, cholestanol, zymostenol, desmosterol and caspase 1 were also observed in the liver. Incubation of AML12 cells in the presence of lanosterol increased caspase 1. In conclusion, squalene administration in rabbits increases the number of modified APOB-containing lipoproteins, and hepatic cholesterol biosynthesis is linked to caspase 1 probably through lanosterol.

The -514C>T polymorphism in the LIPC gene modifies type 2 diabetes risk through modulation of HDL-cholesterol levels in Mexicans

PURPOSE

Both type 2 diabetes (T2D) and low levels of high-density lipoprotein cholesterol (HDL-C) are very prevalent conditions among Mexicans. Genetic variants in the LIPC gene have been associated with both conditions. This study aimed to evaluate the association of the -514C < T (rs1800588) LIPC gene polymorphism with different metabolic traits, particularly the effects of this polymorphism on HDL-C plasma levels and T2D risk.

METHODS

Mediation analysis was used to assess the direct and indirect effects of the -514C>T LIPC gene variant on HDL-C levels, T2D risk, and body mass index (BMI), in 2105 Mexican mestizo participants. We also assessed the functional effect of the -514C>T LIPC variant on the promoter activity of a reporter gene in the HepG2 cell line.

RESULTS

Direct effects show that the -514C>T LIPC polymorphism is significantly associated with increased HDL-C plasma levels (β = 0.03; p < 0.001). The -514C>T variant resulted in an indirect protective effect on T2D risk through increasing HDL-C levels (β = - 0.03; p < 0.001). Marginal direct association between -514C>T and T2D was found (β = 0.08; p = 0.06). Variables directly influencing T2D status were European ethnicity (β = - 7.20; p < 0.001), age (β = 0.04; p < 0.001), gender (β = - 0.15; p = 0.017) and HDL-C (β = - 1.07; p < 0.001). In addition, we found that the -514C>T variant decreases the activity of LIPC promoter by 90% (p < 0.001).

CONCLUSIONS

The -514C>T polymorphism was not directly associated with T2D risk. HDL-C acts as a mediator between -514C>T LIPC gene variant and T2D risk in the Mexican population.

Novel Role for Tranilast in Regulating NLRP3 Ubiquitination, Vascular Inflammation, and Atherosclerosis

Background

Aberrant activation of the NLRP3 (nucleotide‐binding oligomerization domain, leucine‐rich repeat–containing receptor family pyrin domain‐containing 3) inflammasome is thought to play a causative role in atherosclerosis. NLRP3 is kept in an inactive ubiquitinated state to avoid unwanted NLRP3 inflammasome activation. This study aimed to test the hypothesis that pharmacologic manipulating of NLRP3 ubiquitination blunts the assembly and activation of the NLRP3 inflammasome and protects against vascular inflammation and atherosclerosis. Since genetic studies yielded mixed results about the role for this inflammasome in atherosclerosis in low‐density lipoprotein receptor– or apolipoprotein E–deficient mice, this study attempted to clarify the discrepancy with the pharmacologic approach using both models.

Methods and Results

We provided the first evidence demonstrating that tranilast facilitates NLRP3 ubiquitination. We showed that tranilast restricted NLRP3 oligomerization and inhibited NLRP3 inflammasome assembly. Tranilast markedly suppressed NLRP3 inflammasome activation in low‐density lipoprotein receptor– and apolipoprotein E–deficient macrophages. Through reconstitution of the NLRP3 inflammasome in human embryonic kidney 293T cells, we found that tranilast directly limited NLRP3 inflammasome activation. By adopting different regimens for tranilast treatment of low‐density lipoprotein receptor– and apolipoprotein E–deficient mice, we demonstrated that tranilast blunted the initiation and progression of atherosclerosis. Mice receiving tranilast displayed a significant reduction in atherosclerotic lesion size, concomitant with a pronounced decline in macrophage content and expression of inflammatory molecules in the plaques compared with the control group. Moreover, tranilast treatment of mice substantially hindered the expression and activation of the NLRP3 inflammasome in the atherosclerotic lesions.

Conclusions

Tranilast potently enhances NLRP3 ubiquitination, blunts the assembly and activation of the NLRP3 inflammasome, and ameliorates vascular inflammation and atherosclerosis in both low‐density lipoprotein receptor– and apolipoprotein E–deficient mice.

Do the Apoe-/- and Ldlr-/- Mice Yield the Same Insight on Atherogenesis?

Murine models of atherosclerosis are useful for investigating the environmental and genetic influences on lesion formation and composition. Apoe(-/-) and Ldlr(-/-) mice are the 2 most extensively used models. The models differ in important ways with respect to the precise mechanism by which their absence enhances atherosclerosis, including differences in plasma lipoproteins. The majority of the gene function studies have utilized only 1 model, with the results being generalized to atherogenic mechanisms. In only a relatively few cases have studies been conducted in both atherogenic murine models. This review will discuss important differences between the 2 atherogenic models and will point out studies that have been performed in the 2 models where results are comparable and those where different results were obtained.

Hepatic lipase deficiency produces glucose intolerance, inflammation and hepatic steatosis

Metabolic syndrome and type 2 diabetes mellitus constitute a major problem to global health, and their incidence is increasing at an alarming rate. Non-alcoholic fatty liver disease, which affects up to 90% of obese people and nearly 70% of the overweight, is commonly associated with MetS characteristics such as obesity, insulin resistance, hypertension and dyslipidemia. In the present study, we demonstrate that hepatic lipase (HL)-inactivation in mice fed with a high-fat, high-cholesterol diet produced dyslipidemia including hypercholesterolemia, hypertriglyceridemia and increased non-esterified fatty acid levels. These changes were accompanied by glucose intolerance, pancreatic and hepatic inflammation and steatosis. In addition, compared with WT mice, HL(-/-) mice exhibited enhanced circulating MCP1 levels, monocytosis and higher percentage of CD4+Th17+ cells. Consistent with increased inflammation, livers from HL(-/-) mice had augmented activation of the stress SAPK/JNK- and p38-pathways compared with the activation levels of the kinases in livers from WT mice. Analysis of HL(-/-) and WT mice fed regular chow diet showed dyslipidemia and glucose intolerance in HL(-/-) mice without any other changes in inflammation or hepatic steatosis. Altogether, these results indicate that dyslipidemia induced by HL-deficiency in combination with a high-fat, high-cholesterol diet promotes hepatic steatosis and inflammation in mice which are, at least in part, mediated by the activation of the stress SAPK/JNK- and p38-pathways. Future studies are warranted to asses the viability of therapeutic strategies based on the modulation of these kinases to reduce hepatic steatosis associated to lipase dysfunction.

Hepatic Lipase: a Comprehensive View of its Role on Plasma Lipid and Lipoprotein Metabolism

Hepatic lipase (HL) is a key enzyme catalyzing the hydrolysis of triglycerides (TG) and phospholipids (PLs) in several lipoproteins. It is generally recognized that HL is involved in the remodeling of remnant, low-density lipoprotein (LDL), high-density lipoprotein (HDL) and the production of small, dense low-density lipoproteins (sd-LDLs).On the other hand, it is unclear whether HL accelerates or retards atherosclerosis. From the clinical point of view, HL deficiency may provide useful information on answering this question, but the rarity of this disease makes it impossible to conduct epidemiological study.In this review, we describe a comprehensive and updated view of the clinical significance of HL on lipid and lipoprotein metabolism.

Active autophagy but not lipophagy in macrophages with defective lipolysis

During autophagy, autophagosomes fuse with lysosomes to degrade damaged organelles and misfolded proteins. Breakdown products are released into the cytosol and contribute to energy and metabolic building block supply, especially during starvation. Lipophagy has been defined as the autophagy-mediated degradation of lipid droplets (LDs) by lysosomal acid lipase. Adipose triglyceride lipase (ATGL) is the major enzyme catalyzing the initial step of lipolysis by hydrolyzing triglycerides (TGs) in cytosolic LDs. Consequently, most organs and cells, including macrophages, lacking ATGL accumulate TGs, resulting in reduced intracellular free fatty acid concentrations. Macrophages deficient in hormone-sensitive lipase (H0) lack TG accumulation albeit reduced in vitro TG hydrolase activity. We hypothesized that autophagy is activated in lipase-deficient macrophages to counteract their energy deficit. We therefore generated mice lacking both ATGL and HSL (A0H0). Macrophages from A0H0 mice showed 73% reduced neutral TG hydrolase activity, resulting in TG-rich LD accumulation. Increased expression of cathepsin B, accumulation of LC3-II, reduced expression of p62 and increased DQ-BSA dequenching suggest intact autophagy and functional lysosomes in A0H0 macrophages. Markedly decreased acid TG hydrolase activity and lipid flux independent of bafilomycin A1 treatment, however, argue against effective lysosomal degradation of LDs in A0H0 macrophages. We conclude that autophagy of proteins and cell organelles but not of LDs is active as a compensatory mechanism to circumvent and balance the reduced availability of energy substrates in A0H0 macrophages.

Effects of the absence of apolipoprotein e on lipoproteins, neurocognitive function, and retinal function

IMPORTANCE

The identification of a patient with a rare form of severe dysbetalipoproteinemia allowed the study of the consequences of total absence of apolipoprotein E (apoE).

OBJECTIVES

To discover the molecular basis of this rare disorder and to determine the effects of complete absence of apoE on neurocognitive and visual function and on lipoprotein metabolism.

DESIGN, SETTING, AND PARTICIPANTS

Whole-exome sequencing was performed on the patient's DNA. He underwent detailed neurological and visual function testing and lipoprotein analysis. Lipoprotein analysis was also performed in the Cardiovascular Research Institute, University of California, San Francisco, on blood samples from the proband's mother, wife, 2 daughters, and normolipidemic control participants.

MAIN OUTCOME MEASURES

Whole-exome sequencing, lipoprotein analysis, and neurocognitive function.

RESULTS

The patient was homozygous for an ablative APOE frameshift mutation (c.291del, p.E97fs). No other mutations likely to contribute to the phenotype were discovered, with the possible exception of two, in ABCC2 (p.I670T) and LIPC (p.G137R). Despite complete absence of apoE, he had normal vision, exhibited normal cognitive, neurological, and retinal function, had normal findings on brain magnetic resonance imaging, and had normal cerebrospinal fluid levels of β-amyloid and tau proteins. He had no significant symptoms of cardiovascular disease except a suggestion of myocardial ischemia on treadmill testing and mild atherosclerosis noted on carotid ultrasonography. He had exceptionally high cholesterol content (760 mg/dL; to convert to millimoles per liter, multiply by 0.0259) and a high cholesterol to triglycerides ratio (1.52) in very low-density lipoproteins with elevated levels of small-diameter high-density lipoproteins, including high levels of prebeta-1 high-density lipoprotein. Intermediate-density lipoproteins, low-density lipoproteins, and very low-density lipoproteins contained elevated apoA-I and apoA-IV levels. The patient's apoC-III and apoC-IV levels were decreased in very low-density lipoproteins. Electron microscopy revealed large lamellar particles having electron-opaque cores attached to electron-lucent zones in intermediate-density and low-density lipoproteins. Low-density lipoprotein particle diameters were distributed bimodally.

CONCLUSIONS AND RELEVANCE

Despite a profound effect on lipoprotein metabolism, detailed neurocognitive and retinal studies failed to demonstrate any defects. This suggests that functions of apoE in the brain and eye are not essential or that redundant mechanisms exist whereby its role can be fulfilled. Targeted knockdown of apoE in the central nervous system might be a therapeutic modality in neurodegenerative disorders.

HDL biogenesis, remodeling, and catabolism

In this chapter, we review how HDL is generated, remodeled, and catabolized in plasma. We describe key features of the proteins that participate in these processes, emphasizing how mutations in apolipoprotein A-I (apoA-I) and the other proteins affect HDL metabolism. The biogenesis of HDL initially requires functional interaction of apoA-I with the ATP-binding cassette transporter A1 (ABCA1) and subsequently interactions of the lipidated apoA-I forms with lecithin/cholesterol acyltransferase (LCAT). Mutations in these proteins either prevent or impair the formation and possibly the functionality of HDL. Remodeling and catabolism of HDL is the result of interactions of HDL with cell receptors and other membrane and plasma proteins including hepatic lipase (HL), endothelial lipase (EL), phospholipid transfer protein (PLTP), cholesteryl ester transfer protein (CETP), apolipoprotein M (apoM), scavenger receptor class B type I (SR-BI), ATP-binding cassette transporter G1 (ABCG1), the F1 subunit of ATPase (Ecto F1-ATPase), and the cubulin/megalin receptor. Similarly to apoA-I, apolipoprotein E and apolipoprotein A-IV were shown to form discrete HDL particles containing these apolipoproteins which may have important but still unexplored functions. Furthermore, several plasma proteins were found associated with HDL and may modulate its biological functions. The effect of these proteins on the functionality of HDL is the topic of ongoing research.

Animal models of atherosclerosis

In this mini-review several commonly used animal models of atherosclerosis have been discussed. Among them, emphasis has been made on mice, rabbits, pigs and non-human primates. Although these animal models have played a significant role in our understanding of induction of atherosclerotic lesions, we still lack a reliable animal model for regression of the disease. Researchers have reported several genetically modified and transgenic animal models that replicate human atherosclerosis, however each of current animal models have some limitations. Among these animal models, the apolipoprotein (apo) E-knockout (KO) mice have been used extensively because they develop spontaneous atherosclerosis. Furthermore, atherosclerotic lesions developed in this model depending on experimental design may resemble humans’ stable and unstable atherosclerotic lesions. This mouse model of hypercholesterolemia and atherosclerosis has been also used to investigate the impact of oxidative stress and inflammation on atherogenesis. Low density lipoprotein (LDL)-r-KO mice are a model of human familial hypercholesterolemia. However, unlike apo E-KO mice, the LDL-r-KO mice do not develop spontaneous atherosclerosis. Both apo E-KO and LDL-r-KO mice have been employed to generate other relevant mouse models of cardiovascular disease through breeding strategies. In addition to mice, rabbits have been used extensively particularly to understand the mechanisms of cholesterol-induced atherosclerosis. The present review paper details the characteristics of animal models that are used in atherosclerosis research.

Rapid development of atherosclerosis in the world's smallest Microminipig fed a high-fat/high-cholesterol diet

AIM

Experimental studies of human atherogenesis require an appropriate animal model that mimics human physiology and pathology. Because swine physiology is similar to human physiology, we developed a hyperlipidemia-induced atherosclerosis model using the recently developed world's smallest Microminipig(TM).

METHODS

These animals weigh only 5kg at 3months of age, much smaller than any other miniature pig. We found that the administration of a high-fat/high-cholesterol diet containing at least 0.2% cholesterol without cholic acid for as little as eight weeks induces hypercholesterolemia and subsequent atherosclerosis in these animals.

RESULTS

The serum levels of low-density lipoprotein cholesterol(LDL-C) and the percent distribution of cholesterol in the LDL fractions were markedly increased. The hepatic expression of LDL receptor and hydroxymethylglutaryl-CoA reductase was coordinately decreased. The cholesteryl ester transfer protein activity, which plays a role in reverse cholesterol transport, was detected in the serum of the Microminipigs. Niemann-Pick C1-like 1 protein was expressed in both the liver and small intestine; however, hepatic apoB mRNA editing enzyme was not expressed. As in humans, and in contrast to that observed in mice, most of the hepatic lipase activity was localized in the liver. These results suggest that the hyperlipidemia-induced gene expression profile linked to cholesterol homeostasis and atherogenesis is similar in Microminipigs and humans.

CONCLUSION

We conclude that the characteristics of the Microminipig, including its easy handling size, make it an appropriate model for studies of atherosclerosis and related conditions.

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

Genetic basis of atherosclerosis: insights from mice and humans

Atherosclerosis is a complex and heritable disease involving multiple cell types and the interactions of many different molecular pathways. The genetic and molecular mechanisms of atherosclerosis have, in part, been elucidated by mouse models; at least 100 different genes have been shown to influence atherosclerosis in mice. Importantly, unbiased genome-wide association studies have recently identified a number of novel loci robustly associated with atherosclerotic coronary artery disease. Here, we review the genetic data elucidated from mouse models of atherosclerosis, as well as significant associations for human coronary artery disease. Furthermore, we discuss in greater detail some of these novel human coronary artery disease loci. The combination of mouse and human genetics has the potential to identify and validate novel genes that influence atherosclerosis, some of which may be candidates for new therapeutic approaches.

Animal models of atherosclerosis

Atherosclerosis is a chronic inflammatory disorder that is the underlying cause of most cardiovascular disease. Both cells of the vessel wall and cells of the immune system participate in atherogenesis. This process is heavily influenced by plasma lipoproteins, genetics, and the hemodynamics of the blood flow in the artery. A variety of small and large animal models have been used to study the atherogenic process. No model is ideal as each has its own advantages and limitations with respect to manipulation of the atherogenic process and modeling human atherosclerosis or lipoprotein profile. Useful large animal models include pigs, rabbits, and nonhuman primates. Due in large part to the relative ease of genetic manipulation and the relatively short time frame for the development of atherosclerosis, murine models are currently the most extensively used. Although not all aspects of murine atherosclerosis are identical to humans, studies using murine models have suggested potential biological processes and interactions that underlie this process. As it becomes clear that different factors may influence different stages of lesion development, the use of mouse models with the ability to turn on or delete proteins or cells in tissue specific and temporal manner will be very valuable.

Role of hepatic lipase and endothelial lipase in high-density lipoprotein-mediated reverse cholesterol transport

Reverse cholesterol transport (RCT) constitutes a key part of the atheroprotective properties of high-density lipoproteins (HDL). Hepatic lipase (HL) and endothelial lipase (EL) are negative regulators of plasma HDL cholesterol levels. Although overexpression of EL decreases overall macrophage-to-feces RCT, knockout of both HL and EL leaves RCT essentially unaffected. With respect to important individual steps of RCT, current data on the role of EL and HL in cholesterol efflux are not conclusive. Both enzymes increase hepatic selective cholesterol uptake; however, this does not translate into altered biliary cholesterol secretion, which is regarded the final step of RCT. Also, the impact of HL and EL on atherosclerosis is not clear cut; rather it depends on respective experimental conditions and chosen models. More mechanistic insights into the diverse biological properties of these enzymes are therefore required to firmly establish EL and HL as targets for the treatment of atherosclerotic cardiovascular disease.

Animal models for the atherosclerosis research: a review

Atherosclerosis is a leading cause of death worldwide, and its mechanisms are still unclear. However, various animal models have significantly advanced our understanding of the mechanisms involved in atherosclerosis and have allowed the evaluation of therapeutic options. The aim of this paper is to review those animal models (i.e., rabbits, mice, rats, guinea pigs, hamsters, avian, carnivores, swine, and, non-human primates) that have been used to study atherosclerosis. Though there is no single perfect animal model that completely replicates the stages of human atherosclerosis, cholesterol feeding and mechanical endothelial injury are two common features shared by most models of atherosclerosis. Further, with the development of genetically modified animals, these models are significantly broadening our understanding of the pathogenesis of atherosclerosis.

Secretion of triacylglycerol-poor VLDL particles from McA-RH7777 cells expressing human hepatic lipase

Hepatic lipase (HL) plays a role in the catabolism of apolipoprotein (apo)B-containing lipoproteins through its lipolytic and ligand-binding properties. We describe a potential intracellular role of HL in the assembly and secretion of VLDL. Transient or stable expression of HL in McA-RH7777 cells resulted in decreased (by 40%) incorporation of [(3)H]glycerol into cell-associated and secreted triacylglycerol (TAG) relative to control cells. However, incorporation of [(35)S]methionine/cysteine into cell and medium apoB-100 was not decreased by HL expression. The decreased (3)H-TAG synthesis/secretion in HL expressing cells was not attributable to decreased expression of genes involved in lipogenesis. Fractionation of medium revealed that the decreased [(3)H]TAG from HL expressing cells was mainly attributable to decreased VLDL. Expression of catalytically-inactive HL (HL(SG)) (Ser-145 at the catalytic site was substituted with Gly) in the cells also resulted in decreased secretion of VLDL-[(3)H]TAG. Examination of lumenal contents of microsomes showed a 40% decrease in [(3)H]TAG associated with lumenal lipid droplets in HL or HL(SG) expressing cells as compared with control. The microsomal membrane-associated [(3)H]TAG was decreased by 50% in HL expressing cells but not in HL(SG) expressing cells. Thus, expression of HL, irrespective of its lipolytic function, impairs formation of VLDL precursor [(3)H]TAG in the form of lumenal lipid droplets. These results suggest that HL expression in McA-RH7777 cells result in secretion of [(3)H]TAG-poor VLDL.

Impact of combined deficiency of hepatic lipase and endothelial lipase on the metabolism of both high-density lipoproteins and apolipoprotein B-containing lipoproteins

RATIONALE

Hepatic lipase (HL) and endothelial lipase (EL) are extracellular lipases that both hydrolyze triglycerides and phospholipids and display potentially overlapping or complementary roles in lipoprotein metabolism.

OBJECTIVE

We sought to dissect the overlapping roles of HL and EL by generating mice deficient in both HL and EL (HL/EL-dko) for comparison with single HL-knockout (ko) and EL-ko mice, as well as wild-type mice.

METHODS AND RESULTS

Reproduction and viability of the HL/EL-dko mice were impaired compared with the single-knockout mice. The plasma levels of total cholesterol, high-density lipoprotein (HDL) cholesterol, non-HDL cholesterol, and phospholipids in the HL/EL-dko mice were markedly higher than those in the single-knockout mice. Most notably, the HL/EL-dko mice exhibited an unexpected substantial increase in small low-density lipoproteins. Kinetic studies with [(3)H]cholesteryl ether-labeled very-low-density lipoproteins demonstrated that the HL/EL-dko mice accumulated counts in the smallest low-density lipoprotein-sized fractions, as assessed by size exclusion chromatography, suggesting that it arises from lipolysis of very-low-density lipoproteins. HDL from all 3 lipase knockout models had an increased cholesterol efflux capacity but reduced clearance of HDL cholesteryl esters versus control mice. Despite their higher HDL cholesterol levels, neither HL-ko, EL-ko, nor HL/EL-dko mice demonstrated an increased rate of macrophage reverse cholesterol transport in vivo.

CONCLUSIONS

These studies reveal an additive effect of HL and EL on HDL metabolism but not macrophage reverse cholesterol transport in mice and an unexpected redundant role of HL and EL in apolipoprotein B lipoprotein metabolism.

The HDL hypothesis: does high-density lipoprotein protect from atherosclerosis?

There is unequivocal evidence of an inverse association between plasma high-density lipoprotein (HDL) cholesterol concentrations and the risk of cardiovascular disease, a finding that has led to the hypothesis that HDL protects from atherosclerosis. This review details the experimental evidence for this "HDL hypothesis". In vitro studies suggest that HDL has a wide range of anti-atherogenic properties but validation of these functions in humans is absent to date. A significant number of animal studies and clinical trials support an atheroprotective role for HDL; however, most of these findings were obtained in the context of marked changes in other plasma lipids. Finally, genetic studies in humans have not provided convincing evidence that HDL genes modulate cardiovascular risk. Thus, despite a wealth of information on this intriguing lipoprotein, future research remains essential to prove the HDL hypothesis correct.

Mouse hepatic lipase alleles with variable effects on lipoprotein composition and size

The structural features responsible for the activities of hepatic lipase (HL) can be clarified by in vivo comparisons of naturally occurring variants. The coding sequence of HL from C57BL/6J (B6) and SPRET/EiJ (SPRET) mice differs by four amino acids (S106N, A156V, L416V, S480T); however, these changes are not predicted to influence HL function. To test for allelic effects, we generated SPRET-HL transgenics with physiological levels of HL mRNA and HL activity that was parallel in female transgenics and about 70% higher in male transgenics, toward tri-[3H]oleate, compared with B6 controls. We found no correlation between activity levels and plasma lipids. However, significant allelic effects on plasma lipids were observed. Compared with B6-HL, SPRET-HL mediated reductions in total cholesterol (TC) and VLDL-, LDL- and HDL-cholesterol and HDL-triglyceride (TG) in fed males, and SPRET-HL decreased total TG and VLDL- and HDL-TG levels in fasted males. Fasted female transgenics had reduced TC compared with controls. We also found allele and sex effects on lipoprotein particle size. Male transgenic mice had increased VLDL and decreased LDL size, and female transgenic mice had decreased HDL size compared with control animals. These findings demonstrate highly divergent effects of naturally occurring HL coding sequence variants on lipid and lipoprotein metabolism.

Novel roles of hepatic lipase and phospholipid transfer protein in VLDL as well as HDL metabolism

OBJECTIVE

Elevated plasma phospholipid transfer protein (PLTP) expression may increase atherosclerosis in mice by reducing plasma HDL and increasing hepatic VLDL secretion. Hepatic lipase (HL) is a lipolytic enzyme involved in several aspects of the same pathways of lipoprotein metabolism. We investigated whether the effects of elevated PLTP activity are compromised by HL deficiency.

METHODS AND RESULTS

HL deficient mice were crossbred with PLTP transgenic (PLTPtg) mice and studied in the fasted state. Plasma triglycerides were decreased in HL deficiency, explained by reduced hepatic triglyceride secretion. In PLTPtg mice, a redistribution of HL activity between plasma and tissue was evident and plasma triglycerides were also decreased. HL deficiency mitigated or even abolished the stimulatory effect of elevated PLTP activity on hepatic triglyceride secretion. HL deficiency had a modest incremental effect on plasma HDL, which remained present in PLTP transgenic/HL(-/-) mice, thereby partially compensating the decrease in HDL caused by elevation of PLTP activity. HDL decay experiments showed that the fractional turnover rate of HDL cholesteryl esters was delayed in HL deficient mice, increased in PLTPtg mice and intermediate in PLTPtg mice in an HL(-/-) background.

CONCLUSIONS

HL affects hepatic VLDL. Elevated PLTP activity lowers plasma HDL-cholesterol by stimulating the plasma turnover and hepatic uptake of HDL cholesteryl esters. HL is not required for the increase in hepatic triglyceride secretion or for the lowering of HDL-cholesterol induced by PLTP overexpression.

Plasma carboxyl ester lipase activity modulates apolipoprotein B-containing lipoprotein metabolism in a transgenic mouse model

Pancreatic carboxyl ester lipase (CEL) is in the plasma of many mammals, including humans and rats, but not mice. In vitro, CEL hydrolyzes cholesterol esters of apolipoprotein B-containing lipoproteins (apo B-Lp). To study the effect of CEL on metabolism of apo B-Lp and atherosclerosis in vivo, apo E-knockout (EKO) mice, which have high plasma levels of apo B-Lp and are prone to atherosclerosis, were made to secrete CEL into plasma by introducing a transgene containing a liver-specific promoter and rat CEL complementary DNA. Plasma CEL activity in EKO-CEL mice was comparable with that found in rats. Evidence of modification of apo B-Lp by plasma CEL in vivo was an increase in the free cholesterol to cholesterol ester ratio of apo B-Lp from mice on chow or a Western-type diet. In addition, plasma total cholesterol levels were elevated in EKO-CEL mice, with the elevation found exclusively in the apo B-Lp fraction. Associated with the increase in steady-state apo B-Lp levels was an increase in the plasma half-life of very low-density lipoproteins (VLDL) in EKO-CEL mice, measured by the clearance rate of injected VLDL. Interestingly, despite the increase of apo B-Lp, the atherosclerotic lesion did not differ between EKO and EKO-CEL mice on a Western-type diet. In summary, our results demonstrate that plasma CEL modulates apo B-Lp metabolism in vivo, resulting in reduced VLDL clearance and elevated plasma cholesterol levels.

High density lipoprotein cholesterol: an evolving target of therapy in the management of cardiovascular disease

Since the pioneering work of John Gofman in the 1950s, our understanding of high density lipoprotein cholesterol (HDL-C) and its relationship to coronary heart disease (CHD) has grown substantially. Numerous clinical trials since the Framingham Study in 1977 have demonstrated an inverse relationship between HDL-C and one’s risk of developing CHD. Over the past two decades, preclinical research has gained further insight into the nature of HDL-C metabolism, specifically regarding the ability of HDL-C to promote reverse cholesterol transport (RCT). Recent attempts to harness HDL’s ability to enhance RCT have revealed the complexity of HDL-C metabolism. This review provides a detailed update on HDL-C as an evolving therapeutic target in the management of cardiovascular disease.

Leukocyte-derived hepatic lipase increases HDL and decreases en face aortic atherosclerosis in LDLr-/- mice expressing CETP

In addition to hepatic expression, cholesteryl ester transfer protein (CETP) and hepatic lipase (HL) are expressed by human macrophages. The combined actions of these proteins have profound effects on HDL structure and function. It is not known how these HDL changes influence atherosclerosis. To elucidate the role of leukocyte-derived HL on atherosclerosis in a background of CETP expression, we studied low density lipoprotein receptor-deficient mice expressing human CETP (CETPtgLDLr -/-) with a leukocyte-derived HL deficiency (HL -/- BM). HL(-/-) bone marrow (BM), CETPtgLDLr(-/-) mice were generated via bone marrow transplantation. Wild-type bone marrow was transplanted into CETPtgLDLr(-/-) mice to generate HL +/+ BM, CETPtgLDLr(-/-) controls. The chimeras were fed a high-fat, high-cholesterol diet for 14 weeks to promote atherosclerosis. In female HL(-/-) BM, CETPtgLDLr(-/-) mice plasma HDL-cholesterol concentration during high-fat feeding was decreased 27% when compared with HL +/+ BM, CETPtgLDLr(-/-) mice (P < 0.05), and this was associated with a 96% increase in en face aortic atherosclerosis (P < 0.05). In male CETPtgLDLr(-/-) mice, leukocyte-derived HL deficiency was associated with a 16% decrease in plasma HDL-cholesterol concentration and a 25% increase in aortic atherosclerosis. Thus, leukocyte-derived HL in CETPtgLDLr(-/-) mice has an atheroprotective role that may involve increased HDL levels.

Atherogenic, enlarged, and dysfunctional HDL in human PLTP/apoA-I double transgenic mice

In low density lipoprotein receptor (LDLR)-deficient mice, overexpression of human plasma phospholipid transfer protein (PLTP) results in increased atherosclerosis. PLTP strongly decreases HDL levels and might alter the antiatherogenic properties of HDL particles. To study the potential interaction between human PLTP and apolipoprotein A-I (apoA-I), double transgenic animals (hPLTPtg/hApoAItg) were compared with hApoAItg mice. PLTP activity was increased 4.5-fold. Plasma total cholesterol and phospholipid were decreased. Average HDL size (analyzed by gel filtration) increased strongly, hPLTPtg/hApoAItg mice having very large, LDL-sized, HDL particles. Also, after density gradient ultracentrifugation, a substantial part of the apoA-I-containing lipoproteins in hPLTPtg/hApoAItg mice was found in the LDL density range. In cholesterol efflux studies from macrophages, HDL isolated from hPLTPtg/hApoAItg mice was less efficient than HDL isolated from hApoAItg mice. Furthermore, it was found that the largest subfraction of the HDL particles present in hPLTPtg/hApoAItg mice was markedly inferior as a cholesterol acceptor, as no labeled cholesterol was transferred to this fraction. In an LDLR-deficient background, the human PLTP-expressing mouse line showed a 2.2-fold increased atherosclerotic lesion area. These data demonstrate that the action of human PLTP in the presence of human apoA-I results in the formation of a dysfunctional HDL subfraction, which is less efficient in the uptake of cholesterol from cholesterol-laden macrophages.

Reduced atherosclerosis in chow-fed mice expressing high levels of a catalytically inactive human hepatic lipase

Increased expression of catalytically inactive hepatic lipase (ciHL) lowers remnants and low-density lipoproteins (LDL) and may reduce atherosclerosis in mice lacking both LDLreceptors (LDLR) and murine (m) HL. However, in a previous study, ciHL expression failed to reduce atherosclerosis but increased liver fat accumulation after a 3-month high-fat diet, suggesting that diet-induced metabolic changes compromised the antiatherogenic effects of ciHL. Therefore, we hypothesized that reduced dietary fat would reduce atherosclerosis in ciHL expressing mice. Mice lacking both LDLR and mHL, alone, or expressing ciHL were fed a low-fat (chow) diet for 9 months to match the cumulative cholesterol exposure resulting from a 3-month high-fat diet. Plasma lipids and lipoproteins as well as atherosclerosis were determined at sacrifice. Also, liver expression of receptors and proteins contributing to cholesterol delivery including the LDLreceptor related protein (LRP), scavenger receptor (SR)-B1 and apoE were determined. At 9 months, ciHL expression reduced plasma cholesterol by approximately 20% and atherosclerosis by 79% (from 2.67+/-0.61% of aortic surface, Ldlr-/-hl-/-, n=9, to 0.55+/-0.32% of aortic surface, Ldlr-/-hl-/-ciHL, n=7, P=0.01). Also, LRP-expression increased approximately 4-fold, whereas SR-B1 and apoE remained unchanged. These results demonstrate that ciHL expression reduces atherosclerosis. Also, these results demonstrate that ciHL increases LRP expression and suggest increased LRP-mediated lipoprotein clearance as a pathway for ciHL-mediated atherosclerosis reduction.

Vascular lipases, inflammation and atherosclerosis

Members of the lipase family that include lipoprotein lipase, hepatic lipase and endothelial cell lipase play a central role in triglyceride and phospholipid hydrolysis. Because the site of action of these lipases is the endothelium, the endothelium is constantly exposed to products of lipolysis. These lipolysis products could elicit pro- or anti-inflammatory effects in endothelial as well as surrounding cells. These effects could be transient or long-term depending on the nutritional state. While lipolysis is per se anti-atherogenic due to its triglyceride lowering activity, it could also be pro-atherogenic due to prolonged exposure of endothelium to lipolysis products. In addition, lipoprotein lipase expressed in macrophages appears to be pro-atherogenic independent of plasma lipoproteins. In this review we summarize these pro- and anti-inflammatory consequences of lipolysis with respect to atherosclerosis.

Lipolytic and ligand-binding functions of hepatic lipase protect against atherosclerosis in LDL receptor-deficient mice

To elucidate the separate contributions of the lipolytic versus ligand-binding functions of hepatic lipase (HL) to lipoprotein metabolism and atherosclerosis, and to investigate the role of the low density lipoprotein receptor (LDLr) in these processes, we compared mice expressing catalytically active HL (HL-WT) with mice expressing inactive HL (HL-S145G) in a background lacking endogenous HL and the LDLr (LDLr-KOxHL-KO). HL-WT and HL-S145G reduced (P < 0.05 for all) cholesterol (55% vs. 20%), non-HDL-cholesterol (63% vs. 22%), and apolipoprotein B (apoB; 34% vs. 16%) by enhancing the catabolism of autologous (125)I-apoB-intermediate density lipoprotein (IDL)/LDL (fractional catabolic rate in day(-1): 6.07 +/- 0.25, LDLr-KOxHL-WT; 4.76 +/- 0.30, LDLr-KOxHL-S145G; 3.70 +/- 0.13, LDLr-KOxHL-KO); HL-WT had a greater impact on the concentration, composition, particle size, and catabolism of apoB-containing lipoproteins (apoB-Lps) and HDL. Importantly, consistent with the changes in apoB-Lps, atherosclerosis in LDLr-KOxHL-KO mice fed a regular chow diet (RCD) was reduced by both HL-WT and HL-S145G (by 71% and 51% in cross-sectional analysis, and by 85% and 67% in en face analysis; P < 0.05 for all). These data identify physiologically relevant but distinct roles for the lipolytic versus ligand-binding functions of HL in apoB-Lp metabolism and atherosclerosis and demonstrate that their differential effects on these processes are mediated by changes in catabolism via non-LDLr pathways. These changes, evident even in the presence of apoE, establish an antiatherogenic role of the ligand-binding function of HL in LDLr-deficient mice.

Hepatic Lipase Deficiency Delays Atherosclerosis, Myocardial Infarction, and Cardiac Dysfunction and Extends Lifespan in SR-BI/Apolipoprotein E Double Knockout Mice

Objective— SR-BI/apolipoprotein (apo) E double knockout (dKO) mice exhibit many features of human coronary heart disease (CHD), including occlusive coronary atherosclerosis, cardiac hypertrophy, myocardial infarctions, and premature death. Here we determined the effects on this pathology of hepatic lipase (HL) deficiency, which has been shown to significantly modulate atherosclerosis.

Method and Results— The SR-BI/apoE/HL triple knockout (tKO) mice generated for this study lived significantly longer (37%) than corresponding dKO controls (average lifespans: 63.0±0.8 versus 46.0±0.3 days), despite their increased plasma cholesterol levels. At 6 weeks of age, compared with dKO mice, tKOs exhibited significantly less aortic root and coronary artery occlusive atherosclerosis, and improved cardiac structure and function. However, by 9 weeks of age the hearts of tKO mice exhibited lipid-rich coronary occlusions, myocardial infarctions, and cardiac dysfunction essentially identical to that of 6-week-old dKO mice.

Conclusions— HL-deficiency delays the onset and/or progression of atherosclerosis via a SR-BI–independent mechanism. Extent of occlusive coronary arterial lesions was more closely associated with cardiac dysfunction and lifespan than the amount of aortic root atherosclerosis, suggesting that these occlusions in dKO mice are responsible for ischemia, myocardial infarctions, and premature death.

Role of apoA-I, ABCA1, LCAT, and SR-BI in the biogenesis of HDL

The concentration, composition, shape, and size of plasma high-density lipoprotein (HDL) are determined by numerous proteins that influence its biogenesis, remodeling, and catabolism. The discoveries of the HDL receptor (scavenger receptor class B type I, SR-BI) and the ABCA1 (ATP-binding cassette transporter A1) lipid transporter provided two missing links that were necessary to understand the biogenesis and some of the functions of HDL. Existing data indicate that functional interactions between apoA-I and ABCA1 are necessary for the initial lipidation of apoA-I. Through a series of intermediate steps, lipidated apoA-I proceeds to form discoidal HDL particles that can be converted to spherical particles by the action of lecithin:cholesterol acyltransferase (LCAT). Discoidal and spherical HDL can interact functionally with SR-BI and these interactions lead to selective lipid uptake and net efflux of cholesterol and thus remodel HDL. Defective apoA-I/ABCA1 interactions prevent lipidation of apoA-I that is necessary for the formation of HDL particles. In the same way, specific mutations in apoA-I or LCAT prevent the conversion of discoidal to spherical HDL particles. The interactions of lipid-bound apoA-I with SR-BI are affected in vitro by specific mutations in apoA-I or SR-BI. Furthermore, deficiency of SR-BI affects the lipid and apolipoprotein composition of HDL and is associated with increased susceptibility to atherosclerosis. Here we review the current status of the pathway of HDL biogenesis and mutations in apoA-I, ABCA1, and SR-BI that disrupt different steps of the pathway and may lead to dyslipidemia and atherosclerosis in mouse models. The phenotypes generated in experimental mouse models for apoA-I, ABCA1, LCAT, SR-BI, and other proteins of the HDL pathway may facilitate early diagnosis of similar phenotypes in the human population and provide guidance for proper treatment.

Endothelial lipase modulates HDL but has no effect on atherosclerosis development in apoE−/− and LDLR−/− mice

Endothelial lipase (EL) is a determinant of high density lipoprotein-cholesterol (HDL-C) level, which is negatively correlated with atherosclerosis susceptibility. We found no difference in aortic atherosclerotic lesion areas between 26-week-old EL+/+ apolipoprotein E-deficient (apoE-/-) and EL-/- apoE-/- mice. To more firmly establish the role of EL in atherosclerosis, we extended our study to EL-/- and EL+/+ low density lipoprotein receptor-deficient (LDLR-/-) mice that were fed a Western diet. Morphometric analysis again revealed no difference in atherosclerosis lesion area between the two groups. Compared with EL+/+ mice, we found increased HDL-C in EL-/- mice with apoE-/- or LDLR-/- background but no difference in macrophage content between lesions of EL-/- and EL+/+ mice in apoE-/- or LDLR-/- background. EL inactivation had no effect on hepatic mRNAs of proteins involved in reverse cholesterol transport. A survey of lipid homeostasis in EL+/+ and EL-/- macrophages revealed that oxidized LDL-induced ABCA1 was attenuated in EL-/- macrophages. This potentially proatherogenic change may have nullified any minor protective increase of HDL in EL-/- mice. Thus, although EL modulated lipoprotein profile in mice, there was no effect of EL inactivation on atherosclerosis development in two hyperlipidemic atherosclerosis-prone mouse models.

What is so special about apolipoprotein AI in reverse cholesterol transport?

An initial step in reverse cholesterol transport is the movement of unesterified cholesterol from peripheral cells to high-density lipoproteins (HDLs). This transfer usually occurs in extracellular spaces, such as the subendothelial space of a vessel wall, and is promoted by the interaction of lipid-free or lipid-poor apolipoprotein (apo)AI with ATP binding cassette A1 cellular transporters on macrophages (MPhi). Because HDL does not interact with MPhi ATP binding cassette A1 and apoAI is not synthesized by macrophages, this apoAI must be generated from spherical HDL. In this brief review, we propose that spherical apoAI is derived from HDL by remodeling events that are accomplished by proteins secreted by cholesteryl ester-loaded foam cells, including the lipid transfer proteins, phospholipid transfer protein, and cholesteryl ester transfer protein, and the triglyceride hydrolases hepatic lipase and lipoprotein lipase.

Severe hypertriglyceridemia in human APOC1 transgenic mice is caused by apoC-I-induced inhibition of LPL

Studies in humans and mice have shown that increased expression of apolipoprotein C-I (apoC-I) results in combined hyperlipidemia with a more pronounced effect on triglycerides (TGs) compared with total cholesterol (TC). The aim of this study was to elucidate the main reason for this effect using human apoC-I-expressing (APOC1) mice. Moderate plasma human apoC-I levels (i.e., 4-fold higher than human levels) caused a 12-fold increase in TG, along with a 2-fold increase in TC, mainly confined to VLDL. Cross-breeding of APOC1 mice on an apoE-deficient background resulted in a marked 55-fold increase in TG, confirming that the apoC-I-induced hyperlipidemia cannot merely be attributed to blockade of apoE-recognizing hepatic lipoprotein receptors. The plasma half-life of [3H]TG-VLDL-mimicking particles was 2-fold increased in APOC1 mice, suggesting that apoC-I reduces the lipolytic conversion of VLDL. Although total postheparin plasma LPL activity was not lower in APOC1 mice compared with controls, apoC-I was able to dose-dependently inhibit the LPL-mediated lipolysis of [3H]TG-VLDL-mimicking particles in vitro with a 60% efficiency compared with the main endogenous LPL inhibitor apoC-III. Finally, purified apoC-I impaired the clearance of [3H]TG-VLDL-mimicking particles independent of apoE-mediated hepatic uptake in lactoferrin-treated mice. Therefore, we conclude that apoC-I is a potent inhibitor of LPL-mediated TG-lipolysis.

Molecular cloning of nonsecreted endothelial cell-derived lipase isoforms

To expand our knowledge of factors involved in lipid metabolism in the blood vessel wall, we have cloned unique molecular isoforms of endothelial cell-derived lipase (EDL) (HGMW-approved symbol/LIPG). One isoform encoded a truncated protein (EDL2a) lacking the first 80 amino acid residues of the previously characterized EDL1a isoform, including the signal peptide. A similar second clone (EDL2b) was identified that lacked not only the first 80 amino acids, but also a 74-amino-acid region that encodes a portion of the lid domain. RT-PCR analysis confirmed expression of EDL2a/2b isoforms in several human tissues and cultured cells, including endothelial cells. Western blot and immunofluorescence studies using stable transfectants revealed that EDL2a and EDL2b were localized in the cytosol, while, EDL1a was secreted into the culture medium. Cell extracts of EDL2a/2b transfectants did not have triglyceride or phospholipase activity. Thus endothelial cells express three EDL isoforms, two of which remain intracellular and do not function as lipases.

Endothelial Lipase Modulates Susceptibility to Atherosclerosis in Apolipoprotein-E-deficient Mice

Endothelial lipase (EL) expression correlates inversely with circulating high density lipoprotein (HDL) cholesterol levels in genetic mouse models, and human genetic variation in this locus has been linked to differences in HDL cholesterol levels. These data suggest a role for EL in the development of atherosclerotic vascular disease. To investigate this possibility, LIPG-null alleles were bred onto the apoE knockout background, and the homozygous double knockout animals were characterized. Both apoE knockout and double knockout mice had low HDL cholesterol levels when compared with wild-type mice, but the HDL cholesterol levels of the double knockout mice were higher than those of apoE knockout mice. Atherogenic very low density lipoprotein and intermediate density lipoprotein/low density lipoprotein cholesterol levels of the double knockout mice were also greater than those of the apoE knockout animals. Despite this lipid profile, there was a significant approximately 70% decrease in atherosclerotic disease area in double knockout mice on a regular diet. Immunohistochemistry and protein blot studies revealed increased EL expression in the atherosclerotic aortas of the apoE knockout animals. An observed decrease in macrophage content in vessels lacking EL correlated with ex vivo vascular monocyte adhesion assays, suggesting that this protein can modulate monocyte adhesion and infiltration into diseased tissues. These data suggest that EL may have indirect atherogenic actions in vivo through its effect on circulating HDL cholesterol and direct atherogenic actions through vascular wall processes such as monocyte recruitment and cholesterol uptake.

Divergent Effects of the Catalytic and Bridging Functions of Hepatic Lipase on Atherosclerosis

OBJECTIVE

Increased expression of human hepatic lipase (HL) or a catalytically inactive (ci) HL clears plasma cholesterol in mice deficient in low-density lipoprotein receptors (LDLr) and murine HL. We hypothesized that increased expression of both HL and ciHL reduces atherosclerosis in these mice.

METHODS AND RESULTS

Mice deficient in both LDLr and murine HL, alone or transgenically expressing similar levels of either human HL or ciHL, were fed a high-fat, cholesterol-enriched "Western" diet for 3 months to accelerate the development of atherosclerosis. Levels of plasma lipids, insulin, glucose, and liver enzymes were measured monthly, and aortic atherosclerosis was quantitated after 3 months. Plasma insulin, glucose, and liver enzyme levels did not differ significantly from controls. After 3 months, expression of HL reduced plasma cholesterol by 55% to 65% and reduced atherosclerosis by 40%. Surprisingly, expression of ciHL did not reduce plasma cholesterol or atherosclerosis.

CONCLUSIONS

High levels of HL, but not ciHL, delay the development of atherosclerosis in mice deficient in LDLr and mHL. These studies demonstrate that high levels of catalytically active human hepatic lipase (HL) reduce atherosclerosis, whereas high levels of a catalytically inactive HL do not affect atherosclerosis in mice genetically deficient in low-density lipoprotein receptor and mouse HL.

Double-blind, randomized study comparing the effects of two monophasic oral contraceptives containing ethinylestradiol (20 microg or 30 microg) and levonorgestrel (100 microg or 150 microg) on lipoprotein metabolism

The effects of two monophasic oral contraceptives containing ethinylestradiol 20 microg in combination with levonorgestrel 100 microg (EE20/LNG100) or 30 microg and 150 microg (EE30/LNG150), respectively, on lipoprotein metabolism was investigated in a double-blind, randomized study of 12 treatment cycles in healthy female volunteers. Total triglycerides (+32% to +46%, p < 0.05 in comparison to baseline) increased significantly. Triglycerides were highest after six cycles of treatment, decreasing thereafter. Total cholesterol (+1% to +7%), apolipoprotein (apo) B (+21% to +29%) and low-density lipoprotein (LDL) cholesterol (+7% to +17%) increased slightly. High-density lipoprotein (HDL) cholesterol decreased slightly (-11% and -5%), HDL triglycerides increased (+16% and +26%). Apo AI did not change during the study, suggesting that the molar concentration of HDL particles did not change. Apo E (-23% to -14%) decreased, and there was a transitory decrease of lipoprotein (a). Essentially, there was no difference regarding the changes in lipoprotein metabolism between the two treatment groups. The effects of the two combinations of ethinylestradiol and levonorgestrel on triglyceride-rich lipoproteins appear less pronounced than those produced by preparations containing third-generation progestins. It is not likely that the changes in lipoprotein metabolism brought about by the two preparations will alter the risk of future cardiovascular disease in a clinically relevant fashion.

The ligand-binding function of hepatic lipase modulates the development of atherosclerosis in transgenic mice

To investigate the separate contributions of the lipolytic versus ligand-binding function of hepatic lipase (HL) to plasma lipoprotein metabolism and atherosclerosis, we compared mice expressing catalytically active wild-type HL (HL-WT) and inactive HL (HL-S145G) with no endogenous expression of mouse apoE or HL (E-KO x HL-KO, where KO is knockout). HL-WT and HL-S145G reduced plasma cholesterol (by 40 and 57%, respectively), non-high density lipoprotein cholesterol (by 48 and 61%, respectively), and apoB (by 36 and 44%, respectively) (p < 0.01), but only HL-WT decreased high density lipoprotein cholesterol (by 67%) and apoA-I (by 54%). Compared with E-KO x HL-KO mice, both active and inactive HL lowered the pro-atherogenic lipoproteins by enhancing the catabolism of autologous (125)I-apoB very low density/intermediate density lipoprotein (VLDL/IDL) (fractional catabolic rates of 2.87 +/- 0.04/day for E-KO x HL-KO, 3.77 +/- 0.03/day for E-KO x HL-WT, and 3.63 +/- 0.09/day for E-KO x HL-S145G mice) and (125)I-apoB-48 low density lipoprotein (LDL) (fractional catabolic rates of 5.67 +/- 0.34/day for E-KO x HL-KO, 18.88 +/- 1.72/day for E-KO x HL-WT, and 9.01 +/- 0.14/day for E-KO x HL-S145G mice). In contrast, the catabolism of apoE-free, (131)I-apoB-100 LDL was not increased by either HL-WT or HL-S145G. Infusion of the receptor-associated protein (RAP), which blocks LDL receptor-related protein function, decreased plasma clearance and hepatic uptake of (131)I-apoB-48 LDL induced by HL-S145G. Despite their similar effects on lowering pro-atherogenic apoB-containing lipoproteins, HL-WT enhanced atherosclerosis by up to 50%, whereas HL-S145G markedly reduced aortic atherosclerosis by up to 96% (p < 0.02) in both male and female E-KO x HL-KO mice. These data identify a major receptor pathway (LDL receptor-related protein) by which the ligand-binding function of HL alters remnant lipoprotein uptake in vivo and delineate the separate contributions of the lipolytic versus ligand-binding function of HL to plasma lipoprotein size and metabolism, identifying an anti-atherogenic role of the ligand-binding function of HL in vivo.

Severe hypoalphalipoproteinemia in mice expressing human hepatic lipase deficient in binding to heparan sulfate proteoglycan

Unlike human hepatic lipase (hHL) that is mainly cell surface-anchored via binding to heparan sulfate proteoglycans (HSPG), mouse HL (mHL) has a low affinity to HSPG and thus is largely blood-borne. The reduced HSPG binding of mHL is attributable to the C-terminal amino acids. To determine the functions of HSPG binding of hHL in vivo, we created adenovirus vectors encoding hHL or a chimeric protein (designated hHLmt) in which the C-terminal HSPG-binding sequences were replaced with the corresponding mouse sequences. Injecting hHLmt-expressing virus into C57BL/6J mice (1.8 x 10(10) virus particles/mouse) resulted in a 3-fold increase in pre-heparin HL activity, whereas infection with an identical dose of hHL virus did not change pre-heparin HL activity. In hHLmt-expressing mice, the concentration of total cholesterol and phospholipids was inversely related to the hHL activity in pre-heparin plasma in a dose- and time-dependent manner, and the decrease was mainly attributable to high density lipoproteins (HDL) cholesterol and HDL phospholipids. The expression of hHL exhibited no change in plasma total cholesterol or phospholipid levels as compared with control mice infected with luciferase or injected with saline. The reduced HDL lipids in the hHLmt-expressing mice were accompanied by markedly decreased plasma and hepatic apolipoprotein (apo) A-I. In primary hepatocytes isolated from hHLmt-expressing mice, the concentration of cell-associated and secreted apoA-I was decreased by 2-3-fold as compared with hepatocytes isolated from control mice, whereas the levels of apoB and apoE were unaltered. Infection of primary hepatocytes with hHLmt virus ex vivo also resulted in reduced apoA-I secretion but had no effect on cell-associated apoA-I. These results suggest that expression of HSPG binding-deficient hHL has a profound HDL-lowering effect.