Abundant data indicate that overexpression of apolipoprotein A-I (apoA-I) in mice inhibits atherosclerosis. One mechanism is believed to be promotion of reverse cholesterol transport, but no direct ...proof of this concept exists. We developed a novel approach to trace reverse transport of labeled cholesterol specifically from macrophages to the liver and feces in vivo and have applied this approach to investigate the ability of apoA-I overexpression to promote macrophage-specific reverse cholesterol transport.
J774 macrophages were loaded with cholesterol by incubation with acetylated LDL, labeled with 3H-cholesterol, and then injected intraperitoneally into mice. Plasma and feces were collected at 24 hours and 48 hours, when mice were exsanguinated, tissues were harvested, and all were analyzed for tracer counts. 3H-cholesterol was found in the plasma, liver, and feces. For apoA-I overexpression, mice were injected intravenously with apoA-I adenovirus (1011 particles per animal) 3 days before labeled macrophages were injected. ApoA-I overexpression led to significantly higher 3H-cholesterol in plasma, liver, and feces. The amount of 3H-tracer in the liver was 35% higher (P<0.05) and the 3H-tracer excreted into feces over 48 hours was 63% higher (P<0.05) in apoA-I-expressing mice than in control mice.
Injection of 3H-cholesterol-labeled macrophage foam cells is a method of measuring reverse cholesterol transport specifically from macrophages to feces in vivo, and apoA-I overexpression promotes macrophage-specific reverse cholesterol transport.
Adipose harbors a large depot of free cholesterol. However, a role for adipose in cholesterol lipidation of high-density lipoprotein (HDL) in vivo is not established. We present the first evidence ...that adipocytes support transfer of cholesterol to HDL in vivo as well as in vitro and implicate ATP-binding cassette subfamily A member 1 (ABCA1) and scavenger receptor class B type I (SR-BI), but not ATP-binding cassette subfamily G member 1 (ABCG1), cholesterol transporters in this process.
Cholesterol efflux from wild-type, ABCA1(-/-), SR-BI(-/-), and ABCG1(-/-) adipocytes to apolipoprotein A-I (apoA-I) and HDL3 were measured in vitro. 3T3L1 adipocytes, labeled with (3)H-cholesterol, were injected intraperitoneally into wild-type, apoA-I transgenic, and apoA-I(-/-) mice, and tracer movement onto plasma HDL was monitored. Identical studies were performed with labeled wild-type, ABCA1(-/-), or SR-BI(-/-) mouse embryonic fibroblast adipocytes. The effect of tumor necrosis factor-alpha on transporter expression and cholesterol efflux was monitored during adipocyte differentiation. Cholesterol efflux to apoA-I and HDL3 was impaired in ABCA1(-/-) and SR-BI(-/-) adipocytes, respectively, with no effect observed in ABCG1(-/-) adipocytes. Intraperitoneal injection of labeled 3T3L1 adipocytes resulted in increased HDL-associated (3)H-cholesterol in apoA-I transgenic mice but reduced levels in apoA-I(-/-) animals. Intraperitoneal injection of labeled ABCA1(-/-) or SR-BI(-/-) adipocytes reduced plasma counts relative to their respective controls. Tumor necrosis factor-alpha reduced both ABCA1 and SR-BI expression and impaired cholesterol efflux from partially differentiated adipocytes.
These data suggest a novel metabolic function of adipocytes in promoting cholesterol transfer to HDL in vivo and implicate adipocyte SR-BI and ABCA1, but not ABCG1, in this process. Furthermore, adipocyte modulation of HDL may be impaired in adipose inflammatory disease states such as type 2 diabetes mellitus.
HDL cholesterol levels in humans are inversely correlated with the risk of atherosclerosis. The class B scavenger receptor type I (SR-BI) is the first molecularly well-defined HDL receptor, and ...hepatic overexpression of SR-BI in normal mice has been shown to result in decreased plasma HDL cholesterol levels. To determine whether SR-BI overexpression is proatherogenic or is protective against atherosclerosis, LDL receptor-deficient mice were placed on a high-fat/high-cholesterol diet for 2 or 12 weeks to induce atherosclerotic lesions of different stages and then were injected with a recombinant adenovirus encoding murine SR-BI. Transient hepatic overexpression of SR-BI in mice with both early and advanced lesions significantly decreased atherosclerosis. SR-BI expression was associated with markedly decreased HDL cholesterol and either unchanged or only modestly reduced non-HDL cholesterol levels; in all experiments, the mean HDL cholesterol levels were significantly correlated with atherosclerotic lesion size. These data suggest that interventions that promote HDL cholesterol transport and lower plasma HDL cholesterol levels can suppress atherosclerosis, even when initiated after significant lesion development. Thus, stimulation of hepatic SR-BI activity may provide a novel target for therapeutic intervention in atherosclerotic cardiovascular disease.
Endothelial lipase (EL) is a new member of the triglyceride lipase gene family previously reported to have phospholipase activity. Using radiolabeled lipid substrates, we characterized the lipolytic ...activity of this enzyme in comparison to lipoprotein lipase (LPL) and hepatic lipase (HL) using conditioned medium from cells infected with recombinant adenoviruses encoding each of the enzymes. In the absence of serum, EL had clearly detectable triglyceride lipase activity. Both the triglyceride lipase and phospholipase activities of EL were inhibited in a dose-dependent fashion by the addition of serum. The ratio of triglyceride lipase to phospholipase activity of EL was 0.65, compared with ratios of 24.1 for HL and 139.9 for LPL, placing EL at the opposite end of the lipolytic spectrum from LPL. Neither lipase activity of EL was influenced by the addition of apolipoprotein C-II (apoC-II), indicating that EL, like HL, does not require apoC-II for activation. Like LPL but not HL, both lipase activities of EL were inhibited by 1 M NaCl. The relative ability of EL, versus HL and LPL, to hydrolyze lipids in isolated lipoprotein fractions was also examined using generation of FFAs as an end point.
As expected, based on the relative triglyceride lipase activities of the three enzymes, the triglyceride-rich lipoproteins, chylomicrons, VLDL, and IDL, were efficiently hydrolyzed by LPL and HL. EL hydrolyzed HDL more efficiently than the other lipoprotein fractions, and LDL was a poor substrate for all of the enzymes.
The risk of atherosclerosis is inversely associated with plasma levels of high-density lipoprotein cholesterol (HDL-C). However, HDL metabolism is incompletely understood, and there are few effective ...approaches to modulate HDL-C levels. Here we show that inhibition in the liver of the classical proprotein convertases (PCs), but not the atypical PCs S1P and PCSK9, decreases plasma HDL-C levels. This metabolic effect of hepatic PCs is critically dependent on expression of endothelial lipase (EL), an enzyme that directly hydrolyzes HDL phospholipids and promotes its catabolism. Hepatic PCs reduce EL function through direct inactivating cleavage of EL as well as through activating cleavage of angiopoietin-like protein 3 (ANGPTL3), an endogenous inhibitor of EL. Thus, inhibition of hepatic PCs results in increased EL activity, leading to reduced HDL-C as well as impaired reverse cholesterol transport. The hepatic PC–ANGPTL3–EL–HDL pathway is therefore a novel mechanism controlling HDL metabolism and cholesterol homeostasis.
Endothelial lipase (EL) is a recently discovered member of the lipoprotein lipase gene family that hydrolyzes HDL phospholipids ex vivo and reduces HDL cholesterol (HDL-C) levels when overexpressed ...in vivo in mice. To gain further insight into the physiological role of EL in the metabolism of HDL in vivo, studies were performed in which EL was inhibited in wild-type, hepatic lipase knockout (HL(-/-)), and human apoA-I transgenic mice by intravenous infusion of a polyclonal antibody inhibitory to murine EL. As compared with infusion of a control antibody, infusion of the inhibitory antibody resulted in a 25-60% increase in HDL-C levels in the three mouse models, with the peak HDL-C levels occurring at 48 hours after injection. Inhibition of EL also generated larger HDL particles in the HL(-/-) mice. The clearance of HDL phospholipid was significantly slower in human apoA-I transgenic mice injected with an antibody against murine EL (mEL) than in mice injected with a control antibody. We conclude that inhibition of EL results in increased HDL-C levels and that EL is an important enzyme in the physiological regulation of HDL metabolism.
Factors that regulate the metabolism of HDL and apolipoprotein A-I (apoA-I) are incompletely understood. Overexpression of endothelial lipase (EL) markedly reduces plasma levels of HDL cholesterol ...and apoA-I in mice, but the mechanisms of this effect remain unknown.
We used different doses of a recombinant adenoviral vector to overexpress human EL in mice and studied the effects on plasma phospholipase activity, plasma lipids, HDL particle size, HDL turnover, and tissue sites of HDL degradation in mice. Overexpression of EL was associated with a significant dose-dependent increase in postheparin plasma phospholipase activity. Plasma phospholipid, HDL cholesterol, and apoA-I levels were markedly decreased, even at the lowest dose of vector. Kinetic studies demonstrated a significant dose-dependent increase in the fractional catabolic rate of HDL-apolipoprotein in EL-overexpressing mice. The postheparin plasma phospholipase activity was significantly positively correlated with HDL-apolipoprotein fractional catabolic rate. The uptake of apoA-I by the kidney and the liver was significantly increased by 2.5-fold and 3-fold, respectively, in mice overexpressing EL.
Expression of EL in mice results in a dose-dependent increase in postheparin plasma phospholipase activity, catabolic rate of HDL-apolipoprotein, and uptake of apoA-I in both kidney and liver.
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. The cleavage occurs immediately after the sequence RNKR, a known recognition sequence for the proprotein convertase (PC) family. We demonstrate that some PCs, but not all, can proteolytically cleave EL at this site and thereby directly regulate EL enzymatic activity through modulating EL cleavage. Furthermore, specific knockdown of individual PCs proves that PCs are the proteases that cleave EL in human endothelial cells. Interestingly, a homologous site in LPL is also cleaved by PCs. This action is unusual for PCs, which are traditionally known as activators of pro-proteins, and highlights a potential role of PCs in lipid metabolism through their proteolytic processing of lipases.
The endothelium interacts extensively with lipids and lipoproteins, but there are very few data regarding the ability of endothelial cells to secrete lipases. In this study, we investigated the ...ability of endothelial cells to secrete the triglyceride lipase and phospholipase activities characteristic of endothelial lipase (EL), a recently described member of the triglyceride lipase gene family. No lipase activities were detected under basal conditions, but treatment with cytokines significantly stimulated the expression of both activities. Using antibodies to EL, we determined that both activities were primarily a result of this enzyme. In addition to the increase in lipolytic activity, cytokine treatment was demonstrated to substantially upregulate EL protein and EL mRNA in a dose-dependent manner. Cytokines did not change EL mRNA stability. Both new protein synthesis and activation of NF-kappaB influenced the induction of EL by cytokines, suggesting that multiple pathways contribute to this process. The upregulation of EL by cytokines is in sharp contrast to the downregulation by cytokines of the other two major members of this gene family, lipoprotein lipase and hepatic lipase, and has implications for the physiological role of EL in inflammatory conditions and its potential role in the modulation of lipoprotein metabolism during inflammatory conditions, including atherosclerosis.
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. Expression of EL in wild-type Chinese hamster ovary (CHO)-K1 but not in heparan sulfate proteoglycan (HSPG)-deficient CHO-677 cells resulted in 3–4.4-fold increases of 125I-low density lipoprotein (LDL) and 125I-high density lipoprotein 3 binding (HDL3). Inhibition of proteoglycan sulfation by sodium chlorate or incubation of cells with labeled lipoproteins in the presence of heparin (100 μg/ml) abolished bridging effects of EL. An enzymatically inactive EL, EL-S149A, was equally effective in facilitating lipoprotein bridging as native EL. Processing of LDL and HDL differed notably after initial binding via EL to the cell surface. More than 90% of the surface-bound 125I-LDL was destined for internalization and degradation, whereas about 70% of the surface-bound 125I-HDL3 was released back into the medium. These differences were significantly attenuated after HDL clustering was promoted using antibody against apolipoprotein A-I. At equal protein concentration of added lipoproteins the ratio of HDL3 to VLDL bridging via EL was 0.092 compared with 0.174 via HL and 0.002 via LpL. In summary, EL mediates binding and uptake of plasma lipoproteins via a process that is independent of its enzymatic activity, requires cellular heparan sulfate proteoglycans, and is regulated by ligand clustering.
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