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(Arteriosclerosis, Thrombosis, and Vascular Biology. 2000;20:793.)
© 2000 American Heart Association, Inc.

Atherosclerosis and Lipoproteins

In Vivo Evidence for Both Lipolytic and Nonlipolytic Function of Hepatic Lipase in the Metabolism of HDL

K. A. Dugi; M. J. A. Amar; C. C. Haudenschild; R. D. Shamburek; A. Bensadoun; R. F. Hoyt, Jr; J. Fruchart-Najib; Z. Madj; H. B. Brewer, Jr; S. Santamarina-Fojo

From the Molecular Disease Branch (K.A.D., M.J.A.A., C.C.H., R.D.S., H.B.B., S.S.-F.) and Laboratory of Animal Medicine and Surgery (R.F.H.), NHLBI, National Institutes of Health, Bethesda, Md; Cornell University, Ithaca, NY (A.B.); and the Faculté de Pharmacie, Institut Pasteur, INSERM U325, Lille, France (J.F.-N., Z.M.). K.A. Dugi is now at the Department of Internal Medicine I, Endocrinology and Metabolism, Heidelberg University, Heidelberg, Germany.

Correspondence to S. Santamarina-Fojo, National Institutes of Health, Molecular Disease Branch, National Heart, Lung, and Blood Institute, Building 10, Room 7N115, 10 Center Dr, MSC 1666, Bethesda, MD 20892.

	Abstract

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Abstract—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 2x10⁸ plaque-forming units of rHL-AdV and rHL-145G-AdV, similar plasma concentrations were detected in postheparin plasma (HL=8.4±0.8 µg/mL and HL-145G=8.3±0.8 µg/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 ¹²⁵I-labeled apoA-I HDL, ¹³¹I-labeled apoA-II HDL, and [³H]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.

Key Words: hepatic lipase • HDL metabolism • lipolysis • nonlipolytic function

	Introduction

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Hepatic lipase (HL) is a 66-kDa lipolytic enzyme that is synthesized primarily by the liver.¹ It is anchored to the vascular endothelium present in the liver, ovaries, and adrenals via heparan sulfate proteoglycans and can be released into the circulation by the infusion of heparan sulfate.² HL hydrolyzes triglycerides and phospholipids present in chylomicron remnants, IDL, and HDL. Through its function as a lipolytic enzyme, HL plays a major role in the metabolism of circulating plasma lipoproteins.

Several lines of evidence demonstrate the important role of HL in HDL metabolism. Patients with a genetic deficiency of HL have increased plasma levels of HDL cholesterol and phospholipids.^{3 4} Increased HDL is also a hallmark of HL-deficient states induced by infusion of anti-HL antibodies^{5 6} or genetic manipulation⁷ or naturally present in different animal models.⁸ Conversely, increased expression of HL in transgenic mice^{9 10} and rabbits¹¹ significantly decreases plasma HDL concentrations. Similarly, adenovirus-mediated expression of human HL leads to marked reductions in HDL cholesterol^{12 13 14} in different mouse models.

In addition to its enzymatic action as a lipase, recent in vitro studies have suggested a separate role for HL in cellular lipoprotein metabolism by serving as a ligand that mediates the binding and uptake of lipoproteins via proteoglycans and/or receptor pathways. HL enhances the binding and/or uptake of chylomicrons, chylomicron remnants, and ß-VLDL to different cell types in vitro,^{15 16 17 18} a process that appears to require proteoglycans.¹⁷ Most of these studies have investigated the HL-mediated interaction of apolipoprotein (apo)B-containing lipoproteins with cell surface receptors and/or proteoglycans. However, HL may play a similar role in promoting the cellular binding of HDL.^{17 18 19}

In the present article, we investigate, using recombinant adenovirus, potential mechanisms by which HL contributes to the metabolism of HDL in vivo by expressing native HL and catalytically inactive HL (HL-145G) in HL-deficient mice.⁷ Our studies demonstrate that both lipolytic and nonlipolytic pathways are important for normal HDL metabolism in vivo. Thus, we provide, for the first time, in vivo evidence of a role for HL in HDL metabolism independent of lipolysis. In addition, our findings demonstrate that apoA-I HDL catabolism requires the lipolytic function of the enzyme, whereas the catabolism of apoA-II occurs independently of lipolysis, indicating that the metabolism of apoA-I– versus apoA-II–containing HDL may be mediated by different metabolic pathways.

	Methods

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Synthesis of HL-145G cDNA
The catalytically inactive HL-145 was generated by replacing the active serine-145 by glycine (HL-145G) as described.²⁰ The mutant HL-145G cDNA was generated by the overlap extension polymerase chain reaction (PCR)²¹ using oligonucleotide primers made on a DNA synthesizer (model 380B, ABI) and wild-type HL cDNA as template. PCR was performed in an automated DNA thermal cycler (Perkin Elmer Cetus Instruments) using DNA polymerase from Pyrococcus furiosus (Stratagene Inc) and 30 cycles with 1 minute of denaturation at 95°C, 1 minute of annealing at 50°C, and 2 minutes of extension at 72°C in 1x buffer 2 (Stratagene), 200 µmol/L each dATP, dCTP, dGTP, and dTTP (Boehringer Mannheim), and 0.5 µmol/L each primer. The mutant cDNA was subcloned into the pCMV expression vector containing the cytomegalovirus (CMV) immediate early promoter and the polyadenylation site of simian virus 40 (SV40) as previously described²² and amplified with DH-5 cells (BRL). DNA from clones carrying the mutant cDNA was isolated by 1-tube minipreparation and sequenced by the dideoxynucleotide termination method. Recombinant plasmids were purified by cesium chloride double-banding.

In Vitro Transfections
Transfections were performed by the calcium phosphate coprecipitation method²³ by addition of 40 µg of plasmid DNA to each 100-mm plate of subconfluent human embryonal kidney 293 cells (ATCC). Medium for activity determination was harvested 12 to 16 hours after washing and supplemented with glycerol to a final concentration of 30% (vol/vol) and heparin (20 U/mL; Elkins-Sinn). To ensure that HL-145G was catalytically inactive against water and liposoluble triglyceride as well as phospholipid substrates, activity was quantified in triplicate with tributyrin,²⁴ triolein,²⁵ and dioleoylphosphatidylcholine.²⁶

Animals
The HL knockout mice used in these studies were originally generated by Homanics et al.⁷ Control C57BL/6 mice were obtained from Charles River (Wilmington, Mass). For both lines, male animals 3 to 6 months old and weighing 25 to 35 g were used. The mice were bred and housed at the National Institutes of Health under protocols approved by the Animal Care and Use Committee of the National Heart, Lung, and Blood Institute. Animals had free access to food (NIH 07 chow diet, 4.5% fat, 0% cholesterol; Zeigler Brothers Co) and water. The research protocol was approved by the Animal Care and Use Committee of the National Heart, Lung, and Blood Institute of the National Institutes of Health.

Recombinant Adenovirus
The recombinant adenoviruses HL-rAdV, HL-145G-rAdV, and Lucif-rAdV, containing the human HL cDNA,²⁷ the mutant HL-145G cDNA,²⁰ or firefly luciferase,²⁸ respectively, were generated as first described by McGrory et al²⁹ and adapted as published previously.¹³ Briefly, cDNAs were subcloned into a shuttle vector (pAdl2-HL) containing CMV promoter and enhancer elements as well as the SV40 polyadenylation signal.³⁰ Recombinant adenovirus was generated after cotransfection of pAdl2-HL and pJM17 (Ad5 genome) in 293 cells,²⁹ propagated in 293 cells, and purified by cesium chloride density ultracentrifugation. The purified virus was then titered and diluted in 0.2% mouse albumin (Sigma Chemical Co) before infusion into the animals.

Blood Sampling
For all blood sampling, the mice were fasted for 4 hours. Mice were bled from the retro-orbital plexus with capillary tubes coated with heparin (Scientific Products). Coagulation was prevented with EDTA (final concentration of 4 mmol/L). Samples were kept on ice until centrifugation at 2500g for 20 minutes at 4°C. Plasma was removed, divided into aliquots, flash-frozen, and stored at -70°C.

Lipid, Lipoprotein, and Apolipoprotein Quantification
Lipids in 10 µL of mouse plasma were quantified by the COBAS MIRA Plus automated chemistry analyzer (Roche Diagnostic Systems Inc) and enzymatic assays using commercially available kits for total cholesterol (Sigma Diagnostics), free cholesterol, and phospholipids (Wako Chemicals USA Inc). Cholesteryl ester (CE) concentrations were calculated as the difference of total and free cholesterol. HDL cholesterol was determined as the cholesterol remaining in the plasma after precipitation of apolipoprotein B–containing lipoproteins with heparin and calcium. Apolipoproteins A-I and A-II were quantified by sandwich ELISA using polyclonal antibodies raised in rabbits, and purified mouse apoA-I and apoA-II were used as protein standards.

Immunoblotting of ApoA-I and A-II in Mouse Plasma or FPLC Fractions
Twenty microliters of pooled fast protein liquid chromatography (FPLC) fractions (27 to 31 mL) or 1 µL of whole plasma were subjected to SDS-PAGE through 4% to 20% gradient or 10% gels in Tris/glycine buffer (gels and buffer from Novex) and transferred to Immobilon-P membranes (Millipore). Immobilon-P membranes were coupled to polyclonal antibodies to mouse apoA-I or apoA-II raised in rabbits and stained with horseradish peroxidase with the Vectastain ABC kit (Vector Laboratories).

Determination of Lipase Activity, Luciferase Activity, HL Activity, and HL Mass
Postheparin plasma samples were obtained 5 minutes after tail-vein injection of 500 U/kg heparin. Lipase activity was assayed with radiolabeled triolein substrate as described.²⁵ Selective HL activity was quantified in 1 mol/L NaCl. Mouse plasma was diluted up to 100-fold to maintain linearity of the assay. For luciferase assay, tissues were obtained on day 4 after injection with Lucif-rAdV. One hundred milligrams of tissue was homogenized in 0.5 mL of extraction buffer (0.1 mol/L potassium phosphate buffer, pH 7.4, and 1 mmol/L DTT). After 3 freeze/thaw cycles, the homogenates were centrifuged for 20 minutes at 10 000g and 4°C. Thirty microliters of reaction mixture (16.4 mmol/L MgCl₂ and 5.4 mmol/L ATP) was added, and the resulting relative light units were determined with the Monolight luminometer, Analytic Luminescence Laboratory.³¹ HL mass was determined by ELISA with monoclonal antibodies generated against human HL.³²

Fast Protein Liquid Chromatography
Plasma lipoproteins were separated by gel filtration with 2 Superose 6 HR 10/30 columns (Pharmacia-LKB Biotechnology Inc) connected in series. Lipoproteins from 50 µL of plasma were eluted at 0.3 mL/min with PBS buffer containing 1 mmol/L EDTA and 0.02% sodium azide. Lipids in the recovered fractions were assayed as described above. For Western blotting of apolipoproteins, the FPLC fractions were concentrated 15-fold with Ultrafree-MC filters with 10 000 NMWL cellulose membranes (Millipore). FPLC profiles were performed on plasma either from individual mice or pooled from several mice, as stated in the figure legends.

Kinetic Studies
HDL was labeled with [³H]cholesteryl palmityl ether as previously described.³³ In brief, L-phosphatidylcholine type XI-E (Sigma), cholesteryl-1,2-[³H]hexadecyl ether (NEN Life Science Products), and butylated hydroxytoluene (Sigma) (500/1/6, mol/mol/mol) were dried under nitrogen, and then 50 mmol/L Tris, pH 7.4, EDTA 0.01% was added. Liposomes were prepared from these constituents by sonication as described previously³⁴ and were incubated with mouse HDL (1.063<d<1.21 g/mL, 3 mg of total protein) and mouse d>1.21 serum (30 mg of total protein) for 18 hours at 37°C. Labeled HDLs were isolated by sequential ultracentrifugation at 1.063 and 1.21 g/mL density in a TLA-100.2 rotor using a TL-100 ultracentrifuge (Beckman Instruments). Immunoblot analysis, compositional analysis, and FPLC of labeled HDL showed no detectable degradation of apoA-I and apoA-II or alteration in composition and size. Mouse apoA-I and apoA-II were iodinated by a modification of the iodine monochloride method as described previously.³⁵ Approximately 0.5 mol iodine was incorporated per mol protein. Four micrograms of ¹²⁵I-labeled mouse apoA-I or of ¹³¹I-labeled mouse apoA-II was mixed with autologous mouse plasma and dialyzed against sterile PBS containing 0.01% EDTA at 4°C. Radioactivity was quantified in a Packard Cobra-counter (Packard Instrument Co). One million dpm of HDL labeled with [³H]cholesteryl palmityl ether or 5 million dpm ¹²⁵I-labeled mouse apoA-I HDL and ¹³¹I-labeled apoA-II HDL was injected into the saphenous veins of HL-deficient mice. Plasma disappearance curves were generated by dividing the plasma radioactivity at each time point by the plasma radioactivity at the initial time point, which was the same among the study groups (all P>0.4). The fractional catabolic rates (FCRs) for [³H]CE-HDL (10 hours), ¹²⁵I-apoA-I (48 hours), and ¹³¹I-apoA-II HDL (48 hours) were determined from the area under the plasma radioactivity curves by a multiexponential curve-fitting technique on the SAAM program.³⁶ Production rates (PRs) were determined by the formula PR=(apoA-I or apoA-II concentrationxFCRxplasma volume)/body wt. Plasma volume was assumed to be 3.16% of body weight. In a parallel study, plasma and livers were harvested and extracted in 20 volumes of chloroform-methanol, 2:1 (vol/vol); phases were separated by the addition of water, and aliquots of the lower phase were dried and counted in a Tri-Carb 2500 TR liquid scintillation counter (Packard Instrument Co). Mean recoveries at 45 minutes were >95% of injected counts for all groups, with no statistical differences between the groups (both P>0.7).

Statistical Analysis
Data are presented as mean±SD. Comparisons between study groups were performed with unpaired Student’s t test for independent variables and according to Levene’s test for equality of variances. Analyses of mice before and after injection of recombinant adenovirus were done with the paired t test. All calculations were performed with SPSS for Windows, release 5.0, SPSS Inc.

	Results

Top Abstract Introduction Methods Results Discussion References

 
The lipolytic properties of native HL and catalytically inactive HL-145G were first analyzed by in vitro expression in human embryonic kidney 293 cells. These studies confirmed that the mutant HL-145G was completely inactive against water-soluble (tributyrin: HL=96±7 nmol · mL^-1 · h^-1 versus HL-145G=0±0 nmol · mL^-1 · h^-1) and liposoluble (triolein: HL-wild type=4620±90 nmol · mL^-1 · h^-1 versus HL-145G=0±0 nmol · mL^-1 · h^-1) triglycerides. Thus, as expected, replacement of the active serine-145 with glycine abolished HL catalytic activity.

We analyzed the HL concentration and activity in the preheparin and postheparin plasma of HL-deficient mice injected with recombinant adenovirus expressing either luciferase, native HL, or catalytically inactive HL-145G. HL mass and activity were not detected in the preheparin plasma of any of the 3 study mouse groups on either day 0 (data not shown) or day 4 after adenovirus injection, indicating that both the native and mutant human expressed lipases were bound to the mouse vascular endothelium. At day 4 after virus infusion, similar levels of HL were evident in the postheparin plasma of mice injected with rHL-AdV (8.4±0.8 µg/mL) and rHL-145G-AdV (8.3±0.8 µg/mL). As expected, only animals injected with HL-rAdV increased their postheparin plasma HL activity (to 9449 nmol · mL^-1 · min^-1). The specific activity of the expressed human native enzyme (1.1 nmol · ng^-1 · min^-1) was similar to the published specific activity of human HL in postheparin plasma.³⁷

The changes in the plasma cholesterol concentration during the course of the adenovirus study are summarized in Figure 1. Compared with pLucif-AdV–injected mice, animals expressing native HL had a marked decrease in baseline cholesterol values at days 2, 4, and 7 after virus injection (P<0.005 for all). Cholesterol levels also dropped significantly in mice expressing catalytically inactive HL-145G (days 4 and 7; P<0.01 for all). However, the cholesterol reduction induced by catalytically inactive HL-145G was intermediate between that observed in animals expressing native HL and luciferase controls (Figure 1).

View larger version (20K): [in this window] [in a new window]   Figure 1. Plasma cholesterol concentration in mice after infusion of rHL-AdV (n=11), HL-145G-AdV (n=11), or rLucif-AdV (n=8). Values represent mean±SEM before and 2, 4, 8, 10, and 14 days after virus infusion.

The Table summarizes the plasma lipid, lipoprotein, and apolipoprotein concentrations in HL-deficient mice before and 4 days after infusion of recombinant adenovirus. Expression of native HL in HL-deficient mice resulted in significant reductions in the baseline plasma concentrations of total cholesterol (-76±3%, P<0.001), CE (-76±4%, P<0.001), phospholipids (-68±4%, P<0.001), HDL cholesterol (-79±5%, P<0.01), apoA-I (-45±7%, P<0.01), and apoA-II (-59±9%, P<0.05). Similar postheparin plasma concentrations of HL-145G led to less dramatic decreases in total cholesterol (-49±5%, P<0.001), CE (-57±7%, P<0.001), phospholipids (-40%±5%, P<0.001), and HDL cholesterol (-42±5%, P<0.01) than the native enzyme. Interestingly, although apoA-II levels were decreased by -89±9% (P<0.001), plasma concentrations of apoA-I were not significantly altered from baseline values (-21±3%, P=NS). Statistical analysis of the percent reduction of baseline levels of cholesterol, phospholipids, CE, HDL cholesterol, apoA-I, and apoA-II revealed significant (P<0.005) differences between HL knockout mice expressing native-HL versus mutant HL-145G (Table). Injections of rLucif-AdV resulted in significant hepatic luciferase activity, reflecting high-level liver expression (data not shown), but had no impact on the plasma lipid profile of HL-deficient mice.

View this table: [in this window] [in a new window]   Table 1. Lipid Profile in HL-Deficient Mice Before and After Infusion of Recombinant Adenovirus

Analysis of the plasma lipoproteins by FPLC (Figure 2) demonstrated significant reductions in the baseline HDL cholesterol and HDL phospholipid in HL-deficient mice expressing native HL (Figure 2, A and B). Less dramatic decreases were observed in the HDL cholesterol and HDL phospholipid of mice expressing similar concentrations of HL-145G (Figure 2, C and D), suggesting that the lipolytic activity of HL is essential for at least part of its effect on HDL lipid lowering. Immunoblot analysis of pooled HDL FPLC fractions (27 to 31 mL) is shown in Figure 2 (inset). Compared with day 0, both apoA-I and apoA-II were decreased in the HDL fractions of mice expressing native HL. In contrast, only apoA-II appeared to decrease in mice expressing the mutant HL-145G. Animals expressing luciferase control showed no alterations in their baseline lipoprotein profile (Figure 2, E and F).

View larger version (29K): [in this window] [in a new window]   Figure 2. FPLC analysis of the plasma lipoproteins in HL-deficient mice before and 4 days after infusion of recombinant adenovirus. The cholesterol and phospholipid distributions in the plasma lipoproteins of HL-deficient mice before and after injection of recombinant adenovirus expressing native HL (A and B), catalytically inactive HL145-G (C and D), and luciferase (E and F) are illustrated. For each of the 3 study groups, 50 µL of pooled plasma from 5 mice (10 µL each) was applied to the FPLC. Insets, Immunoblot analyses of apoA-I and apoA-II in pooled (27 to 31 mL) FPLC fractions of HDL.

To investigate the mechanism(s) by which HL contributes to the metabolism of HDL in HL-deficient mice and the relative contribution of the lipolytic and ligand-binding roles of HL in this process, the plasma kinetics of [³H]CE HDL, ¹²⁵I-labeled apoA-I HDL, and ¹³¹I-labeled apoA-II HDL was determined 4 days after infusion of recombinant adenovirus. As illustrated in Figure 3, the plasma clearance of [³H]CE HDL was significantly enhanced in mice expressing active HL as well as catalytically inactive HL-145G compared with mice expressing luciferase control (FCRs for HL, 9.3±0.9; HL-145G, 8.3±1.1; and luciferase, 4.1±0.7; P<0.05 for all). Thus, even in the absence of lipolysis, HL enhances the clearance of CE from HDL, suggesting a role of the ligand-binding function of the enzyme in HDL metabolism. However, maximal clearance of CE from HDL occurred with expression of the fully active HL, indicating that both lipolytic and nonlipolytic functions are necessary for HDL metabolism.

View larger version (25K): [in this window] [in a new window]   Figure 3. In vivo metabolism of [3H]CE, 125I-apoA-I, and 131I-apoA-II radiolabeled HDL in HL knockout mice after injection with rHL-AdV, rHL-145G-AdV, and rLucif-AdV. Autologous HL knockout mouse HDL was isolated and radiolabeled as described in Methods. The plasma clearances of [3H]CE HDL (top), 125I-apoA-I HDL (middle), and 131I-apoA-II HDL (bottom) in HL knockout mice 4 days after adenovirus infusion are shown. FCRs are shown in inset.

The plasma clearance of ¹²⁵I-labeled apoA-I in mice expressing native HL was significantly faster than that of mice expressing catalytically inactive HL-145G or luciferase control (FCRs for native HL, HL-145G, and luciferase were 4.6±0.3, 1.4±0.1, and 1.3±0.1; P<0.001 for HL versus HL-145G and luciferase). Thus, expression of active but not catalytically inactive HL significantly enhanced the clearance of apoA-I HDL. The apoA-I PR (mg · kg^-1 · d^-1) was decreased (P<0.007) in mice expressing HL-145G (13.0±0.86) compared with animals expressing HL (29.1±1.6) and luciferase (23±0.7). In contrast, the plasma clearance of apoA-II HDL (Figure 3) was significantly enhanced in HL-deficient mice expressing both native and catalytically inactive HL compared with luciferase controls (FCRs for HL, HL-145G, and luciferase were 5.6±0.5, 4.4±0.2, and 2.1±0.1; P<0.001 for all). These data indicate that HL-mediated lipolysis is essential for the metabolism of apoA-I but not apoA-II–containing HDL, suggesting alternative catabolic pathways for HDL with different apolipoprotein composition. Mice expressing the catalytically inactive HL-145G had significantly decreased (P<0.01) apoA-II PR (5.6±0.2) compared with animals expressing HL (15.8±0.7) and luciferase (20.4±0.6).

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
Previous studies have indicated that HL plays an important role in the metabolism of HDL. The effects of HL on HDL plasma levels and metabolism have been attributed primarily to the function of the enzyme as both an acylglycerol hydrolase and phospholipase. HL enhances the selective uptake of free cholesterol and/or CEs from rat HDL by hepatoma cells,³⁸ transfected CHO cells expressing HL,¹⁹ and transfected rat hepatoma cells.^{17 39 40} The hydrolysis of surface phospholipids may enhance the interaction of HDL apoproteins with membrane sites, thus promoting cellular HDL cholesterol uptake.^{38 41} Thus, phospholipase treatment of HDL increases the uptake of HDL CEs by cultured hepatocytes.⁴¹ The importance of HL phospholipase activity has also been confirmed by rat liver perfusion studies, which demonstrated that HL could not enhance the uptake of CEs from HDL that had been reconstituted with a nonhydrolyzable phospholipid.^{39 40}

Recent in vitro data suggest that in addition to its enzymatic action as a lipase, HL serves as a ligand that mediates the cellular uptake of lipid and/or lipoproteins.^{15 16 17 19 42 43 44} Thus, expression of a cell surface–anchored form of HL in CHO cells¹⁹ as well as a secreted form of HL in human Hep3 hepatoma cells⁴⁵ markedly enhanced the uptake of HDL cholesterol even in the presence of antibodies¹⁹ or tetrahydrolipstatin,⁴⁵ which blocked lipolysis.¹⁹ Ji et al¹⁷ showed that cellular HDL-CE uptake was only partially reduced in hepatoma cells expressing catalytically inactive HL compared with cells expressing the active enzyme. More recently, Lambert et al⁴⁶ demonstrated enhanced scavenger receptor (SR)-B1–mediated cellular selective HDL-CE uptake in the presence of both active and catalytically inactive HL. In this latter in vitro study, both the lipolytic and nonlipolytic functions of HL were necessary for maximal stimulation of HDL-CE selective uptake by HL. These combined studies have provided in vitro evidence supporting an important role for HL independent of lipolysis. However, the physiological relevance of the ligand-binding function of HL and the relative contributions of the lipolytic and nonlipolytic functions of HL in HDL metabolism in vivo have yet to be established.

To gain further insights into the mechanisms by which HL modulates HDL metabolism in vivo, we used recombinant adenovirus to express native HL, catalytically inactive HL-145G, and luciferase control in HL-deficient mice. Expression of catalytically inactive HL-145G led to a reduction of total and HDL cholesterol that was intermediate between that observed in mice expressing similar levels of the native HL and those expressing luciferase control. The lowering of HDL-cholesterol levels in the absence of HL catalytic function provides in vivo evidence of an important role for HL in HDL metabolism independent of lipolysis, consistent with a ligand-binding function. However, maximal HDL-cholesterol lowering was achieved only when the fully active enzyme was expressed, demonstrating that both lipolytic and nonlipolytic functions are necessary for the full effect of HL on HDL metabolism in vivo. These findings differ from those published by Dichek et al,¹⁰ who found that expression of catalytically inactive HL in apoE knockout or apoB transgenic mice had only a minimal effect on HDL plasma levels. In contrast to HL knockout mice, apoB transgenic and apoE knockout mice express endogenous mouse HL. In addition, most of their cholesterol is found in apoB-containing lipoproteins instead of HDL. The presence of endogenous active mouse HL may account for the lack of effect observed with expression of the catalytically inactive enzyme in these two animal models.

Analysis of the changes in the major HDL apolipoproteins in HL knockout mice demonstrated that native and catalytically inactive HL had different effects on apoA-I and apoA-II plasma concentrations. Mice expressing the native enzyme had significant reductions in baseline plasma levels of both apolipoproteins. However, although HL-145G reduced the plasma levels of apoA-II significantly, by 89%, the plasma levels of apoA-I were not significantly decreased. Thus, the changes in the plasma concentrations of apoA-II could be accounted for by the nonlipolytic function of HL, whereas apoA-I lowering required expression of the fully active HL.

Dual-labeled HDL studies in HL-deficient mice after injection of HL-AdV, HL-145G-AdV, and Lucif-AdV demonstrated that compared with luciferase control, native HL significantly enhanced the clearance of apoA-I, apoA-II, and CEs in HDL. These findings are consistent with either HL-facilitated selective removal of HDL-CE followed by separate catabolism of the cholesterol-depleted HDL or HL-enhanced catabolic pathway(s) that may involve, at least in part, whole-particle uptake. The former mechanism could require the interaction of HL with the newly described HDL-CE selective uptake receptor SR-BI.⁴⁷ Thus, recent in vitro studies have demonstrated that HL facilitates the selective uptake of HDL-CE mediated by SR-BI.^{46 47} The cholesterol-depleted HDL could then be further catabolized by receptors such as cubilin, the receptor for the vitamin B₁₂/intrinsic factor, which has also recently been implicated in HDL whole-particle uptake by the kidney.^{48 49} An alternative mechanism could involve HL-facilitated whole-particle catabolism of HDL. Although the hepatic receptor(s) involved in facilitating HDL whole-particle uptake are yet to be elucidated, several studies have suggested that the LDL receptor–related protein as well as surface heparan sulfate proteoglycans may be involved in this process.¹⁷ Indirect evidence implicating LDL receptor–related protein in the whole-particle uptake of apoE-containing HDL has also been described.^{50 51}

In contrast to native HL, mice expressing catalytically inactive HL-145G had enhanced catabolism of apoA-II and HDL-CE but not apoA-I. Thus, lipolysis appears to be necessary for the HL-mediated effect on apoA-I but not for apoA-II or HDL-CE catabolism. Conversely, the ligand-binding function of HL facilitates the clearance of both apoA-II and HDL-CE. These data suggest an alternative catabolic pathway for apoA-II– versus apoA-I–containing HDL. We recently demonstrated that both active HL and catalytically inactive HL-145G enhanced SR-BI–mediated selective uptake of CE from HDL in transfected human embryonic kidney 293 cells.⁴⁶ Consistent with the findings presented in this study, expression of the fully active HL was required for maximal enhancement of SR-BI–mediated HDL-CE selective uptake in 293 cells. However, it is unclear how HL-facilitated selective uptake of HDL-CE mediated by SR-BI could differentially affect apoA-I versus apoA-II catabolism. In vitro studies have shown that both apoA-I and apoA-II can bind to SR-BI,⁵² but to date there are no data supporting a potential difference in the binding affinity of LpA-I/A-II versus LpA-I to SR-BI. A second possibility is that by facilitating the selective removal of CE from HDL by SR-BI, HL also enhances the subsequent shedding of lipid-poor apoA-I and apoA-II HDL. Selective displacement of apoA-II from delipidated HDL followed by catabolism of the apoprotein could account for the preferential reduction of apoA-II plasma levels observed in mice expressing HL-145G. However, there are no data in the literature supporting the preferential displacement of apoA-II versus apoA-I from HDL. In fact, previous studies have indicated that apoA-I is more easily displaced from both rHDL and native HDL than apoA-II.⁵³ However, the possibility that because of its greater hydrophobicity, apoA-II is preferentially displaced from lipid-poor HDL after its interaction with SR-BI has not been adequately explored. Preferential catabolism of apoA-II–containing HDL by the newly described whole-particle HDL receptor in the kidney, cubilin,^{48 49} seems unlikely, because cubilin-mediated catabolism of HDL appears to preferentially affect apoA-I versus apoA-II,⁴⁸ although both apoproteins equally inhibit cubilin-mediated HDL endocytosis.⁴⁹ Increased concentrations of apoA-I– and A-II– versus apoA-I–containing HDL in mouse plasma would also explain the preferential reduction in apoA-II plasma levels in mice expressing HL-145G, even if apoA-I remained the preferred ligand for the various proposed receptors. However, the relative ratio of HDL particles containing either apoA-I or both apoA-I and apoA-II in mice has not been reported.

One other potential explanation for the difference in apoA-I versus apoA-I/A-II HDL catabolism may be found in previous studies, which have shown that compared with apoA-I particles, HL binds more avidly to HDL particles containing apoA-II in vitro.⁵⁴ Hime et al⁵⁵ used reconstituted HDL particles to demonstrate that HL had a higher affinity to apoA-II–containing particles. In contrast, the V_max for phospholipid hydrolysis was greater in apoA-I–containing particles. Because particle binding can be considered a prerequisite for the proposed ligand-binding function of HL, the increased affinity of HL for apoA-II–containing HDL could, in turn, facilitate the preferential interaction of apoA-II–containing HDL with cell surface receptor and proteoglycans. This mechanism would explain, at least in part, the greater reduction in apoA-II versus apoA-I in mice expressing the catalytically inactive HL-145G.

In summary, the present study provides the first in vivo evidence of a role for HL in HDL metabolism independent of lipolysis. We demonstrate that both the lipolytic and nonlipolytic functions of HL are important for normal HDL metabolism in vivo. The plasma clearance of apoA-I HDL requires the lipolytic function of the enzyme, whereas the catabolism of apoA-II HDL occurs independently of lipolysis. Our combined in vivo findings suggest that the lipolytic and ligand-binding mechanisms by which HL influences HDL catabolism may differentially influence the metabolic fate of HDL particles containing apoA-I or apoA-II. These data indicate that the lipolytic and ligand-binding functions of HL may be important in facilitating distinct metabolic pathways involved in the catabolism of apoA-I– versus apoA-II–containing HDL and provides new insights into the role of HL in HDL metabolism in vivo.

Received August 3, 1999; accepted November 5, 1999.

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