Lecithin: cholesterol acyltransferase expression has minimal effects on macrophage reverse cholesterol transport in vivo.

Background: Lecithin:cholesterol acyltransferase (LCAT) catalyzes the formation of plasma cholesteryl ester, plays a key role in high-density lipoprotein metabolism, and has been believed to be critical in the process of reverse cholesterol transport (RCT).

Methods And Results: The role of LCAT in RCT from macrophages was quantified with a validated assay involving intraperitoneal injection in mice of (3)H-cholesterol-labeled J774 macrophages and monitoring the appearance of tracer in plasma, liver, bile, and feces. Human LCAT overexpression in human apolipoprotein A-I transgenic mice substantially increased plasma high-density lipoprotein cholesterol levels but surprisingly did not increase macrophage RCT.

A naturally occurring variant of endothelial lipase associated with elevated HDL exhibits impaired synthesis.

Human endothelial lipase (EL) is a member of a family of lipases and phospholipases that are involved in the metabolism of plasma lipoproteins. EL displays a preference to hydrolyze lipids in HDL. We report here that a naturally occurring low frequency coding variant in the EL gene (LIPG), glycine-26 to serine (G26S), is significantly more common in African-American individuals with elevated HDL cholesterol (HDL-C) levels.

Overexpression of apolipoprotein F reduces HDL cholesterol levels in vivo.

Objective: Apolipoprotein F (ApoF) is a protein component of several lipoprotein classes including HDL. It is also known as lipid transfer inhibitor protein (LTIP) based on its ability to inhibit lipid transfer between lipoproteins ex vivo. We sought to investigate the role of ApoF in HDL metabolism.

Effects of nevirapine and efavirenz on HDL cholesterol levels and reverse cholesterol transport in mice.

Objective: The mechanism by which non-nucleoside reverse transcriptase inhibitors (NNRTIs) increase HDL cholesterol (HDL-C) in HIV+ patients and the benefits of this with respect to cardiovascular risk are not known. Studies were conducted to test the hypothesis that NNRTIs have a beneficial effect on HDL-C and reverse cholesterol transport (RCT).

Methods: LDLr-/- and hA-I transgenic mice were fed a Western diet containing either nevirapine (20mg/kg per day), efavirenz (10mg/kg per day), or diet alone.

Potent and selective PPAR-alpha agonist LY518674 upregulates both ApoA-I production and catabolism in human subjects with the metabolic syndrome.

Objective: The study of PPAR-alpha activation on apoA-I production in humans has been limited to fibrates, relatively weak PPAR-alpha agonists that may have other molecular effects. We sought to determine the effect of a potent and highly specific PPAR-alpha agonist, LY518674, on apoA-I, apoA-II, and apoB-100 kinetics in humans with metabolic syndrome and low levels of HDL cholesterol (C).

Methods And Results: Subjects were randomized to receive LY518674 (100 microg) once daily (n=13) or placebo (n=15) for 8 weeks.

Macrophage ABCA1 and ABCG1, but not SR-BI, promote macrophage reverse cholesterol transport in vivo.

Macrophage ATP-binding cassette transporter A1 (ABCA1), scavenger receptor class B type I (SR-BI), and ABCG1 have been shown to promote cholesterol efflux to extracellular acceptors in vitro and influence atherosclerosis in mice, but their roles in mediating reverse cholesterol transport (RCT) from macrophages in vivo are unknown. Using an assay of macrophage RCT in mice, we found that primary macrophages lacking ABCA1 had a significant reduction in macrophage RCT in vivo, demonstrating the importance of ABCA1 in promoting macrophage RCT, however substantial residual RCT exists in the absence of macrophage ABCA1. Using primary macrophages deficient in SR-BI expression, we found that macrophage SR-BI, which was shown to promote cholesterol efflux in vitro, does not contribute to macrophage RCT in vivo.

Endothelial lipase is less effective at influencing HDL metabolism in vivo in mice expressing apoA-II.

Endothelial lipase (EL) plays an important physiological role in modulating HDL metabolism. Data suggest that plasma contains an inhibitor of EL, and previous studies have suggested that apolipoprotein A-II (apoA-II) inhibits the activity of several enzymes involved in HDL metabolism. Therefore, we hypothesized that apoA-II may reduce the ability of EL to influence HDL metabolism.

Adipocyte enhancer-binding protein 1 is a potential novel atherogenic factor involved in macrophage cholesterol homeostasis and inflammation.

Peroxisome proliferator-activated receptor gamma1 (PPARgamma1) and liver X receptor alpha (LXRalpha) play pivotal roles in macrophage cholesterol homeostasis and inflammation, key biological processes in atherogenesis. Herein we identify adipocyte enhancer-binding protein 1 (AEBP1) as a transcriptional repressor that impedes macrophage cholesterol efflux, promoting foam cell formation, via PPARgamma1 and LXRalpha down-regulation. Contrary to AEBP1 deficiency, AEBP1 overexpression in macrophages is accompanied by decreased expression of PPARgamma1, LXRalpha, and their target genes ATP-binding cassette A1, ATP-binding cassette G1, apolipoprotein E, and CD36, with concomitant elevation in IL-6, TNF-alpha, monocyte chemoattractant protein 1, and inducible NO synthase levels.

Proprotein convertases [corrected] are responsible for proteolysis and inactivation of endothelial lipase.

Plasma lipoprotein metabolism is tightly regulated by several members of the triglyceride lipase family, including endothelial lipase (EL) and lipoprotein lipase (LPL). Our previous work suggested that EL is proteolytically processed. In this report, we have used a combination of epitope tagging, mutagenesis, and N-terminal sequencing to determine the precise location of the cleavage site within EL.

Structural basis of endothelial lipase tropism for HDL.

Lipoprotein lipase (LPL) and endothelial lipase (EL), the most closely related enzymes among the members of the triglyceride lipase gene family with regard to primary sequence, have distinct lipolytic properties (triglyceride lipase vs. phospholipase) as well as different preferences for specific types of lipoproteins [triglyceride-rich lipoproteins vs. high density lipoprotein (HDL)] Lipid substrate specificity is believed to be conferred by the lid region located in the amino-terminal domain of the enzymes, whereas surprisingly little work has been done to identify the region mediating lipoprotein substrate specificity.

Role of N-linked glycosylation in the secretion and activity of endothelial lipase.

Human endothelial lipase (EL), a member of the triglyceride lipase gene family, has five potential N-linked glycosylation sites, two of which are conserved in both lipoprotein lipase and hepatic lipase. Reduction in molecular mass of EL after treatment with glycosidases and after treatment of EL-expressing cells with the glycosylation inhibitor tunicamycin demonstrated that EL is a glycosylated protein. Each putative glycosylation site was examined by site-directed mutagenesis of the asparagine (Asn).

Endothelial lipase promotes the catabolism of ApoB-containing lipoproteins.

Endothelial lipase (EL) has been found to be a key enzyme in high-density lipoprotein (HDL) metabolism in mice, leading to the concept that inhibition of EL could be a novel strategy for raising HDL cholesterol levels. However, mice are "HDL animals" and the effect of EL on atherogenic apoB-containing lipoproteins has not been elucidated. We previously found that EL is capable of hydrolyzing very low-density lipoprotein (VLDL) and LDL lipids ex vivo.

Endogenously produced endothelial lipase enhances binding and cellular processing of plasma lipoproteins via heparan sulfate proteoglycan-mediated pathway.

Endothelial lipase (EL) is a new member of the triglyceride lipase gene family, which includes lipoprotein lipase (LpL) and hepatic lipase (HL). Enzymatic activity of EL has been studied before. Here we characterized the ability of EL to bridge lipoproteins to the cell surface.

Normal production rate of apolipoprotein B in LDL receptor-deficient mice.

The low density lipoprotein (LDL) receptor is well known for its role in mediating the removal of apolipoprotein B (apoB)-containing lipoproteins from plasma. Results from in vitro studies in primary mouse hepatocytes suggest that the LDL receptor may also have a role in the regulation of very low density lipoprotein (VLDL) production. We conducted in vivo experiments using LDLR-/-, LDLR+/-, and wild-type mice (LDLR indicates LDL receptor gene) in which the production rate of VLDL was measured after the injection of [35S]methionine and the lipase inhibitor Triton WR1339.