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

Abstract

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 2ϫ10 8 plaque-forming units of rHL-AdV and rHL-145G-AdV, similar plasma concentrations were detected in postheparin plasma (HLϭ8.4Ϯ0.8 g/mL and HL-145Gϭ8.3Ϯ0.8 g/mL). Mice expressing HL had significant reductions of cholesterol (Ϫ76%), phospholipids (PL; Ϫ68%), HDL cholesterol (Ϫ79%), apolipoprotein (apo) A-I (Ϫ45%), and apoA-II (Ϫ59%; PϽ0.05 for all), whereas mice expressing HL-145G decreased their cholesterol (Ϫ49%), PL (Ϫ40%), HDL cholesterol (Ϫ42%), and apoA-II (Ϫ89%; PϽ0.005 for all) but had no changes in apoA-I. The plasma kinetics of 125 I-labeled apoA-I HDL, 131 I-labeled apoA-II HDL, and Key Words: hepatic lipase Ⅲ HDL metabolism Ⅲ lipolysis Ⅲ nonlipolytic function H epatic 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 HLdeficient states induced by infusion of anti-HL antibodies 5,6 or genetic manipulation 7 or naturally present in different animal models. 8 Conversely, increased expression of HL in transgenic mice 9,10 and rabbits 11 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 ␤-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)Bcontaining 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...

The Ligand-binding Function of Hepatic Lipase Modulates the Development of Atherosclerosis in Transgenic Mice

“…Mice and Diets-The generation of pCMV vectors containing wildtype and catalytically inactive human HL cDNAs has been described previously (25,28). A 1.6-kb fragment of human HL cDNA was excised from the pCMV vector and subcloned into the expression plasmid pLIV.11 (39) modified by the addition of NotI linkers.…”

“…Total cholesterol (TC), TG, PL, and free cholesterol (FC) in plasma samples were analyzed using enzyme kits (Sigma, St. Louis, MO) as described (45). HDL-C was determined as the cholesterol remaining after precipitation of apoB-Lp with dextran sulfate (Bayer Corp., Tarrytown, NY) as described (28). Non-HDL-C was obtained by subtraction of the HDL-C from the TC.…”

“…Lipids in the recovered fractions were analyzed using enzyme kits as described (25,28). The elution volumes of the plasma lipoproteins separated by FPLC were as follows: VLDL, 15.0 -17.0 ml; IDL/LDL, 19.0 -23.0 ml; and HDL, 29.0 -31.0 ml.…”

“…Decreased apoB-Lp levels are also observed in transgenic mice that overexpress catalytically inactive HL in LDLR knockout (KO) mice (27). Finally, adenoviral expression of HL-S145G in mice with no endogenous expression of HL (HL-KO) decreased the plasma concentrations of HDL-C by mechanisms independent of lipolysis (28). However, these in vivo reports are limited by the confounding effects of having fully active, endogenous mouse HL present along with HL-S145G in the adenovirus and transgenic studies in E-KO mice and by non-steady-state expression of catalytically inactive HL-S145G in the adenovirus studies (25,26,28).…”

“…Finally, adenoviral expression of HL-S145G in mice with no endogenous expression of HL (HL-KO) decreased the plasma concentrations of HDL-C by mechanisms independent of lipolysis (28). However, these in vivo reports are limited by the confounding effects of having fully active, endogenous mouse HL present along with HL-S145G in the adenovirus and transgenic studies in E-KO mice and by non-steady-state expression of catalytically inactive HL-S145G in the adenovirus studies (25,26,28). More recently, Dichek et al (29) reported that overexpression of catalytically inactive HL in LDLR-KO, LDLR-KO ϫ apoB-100, and LDLR-KO ϫ apoB-48 (HL-KO) mice facilitates the clearance of apoB-48-and apoB-100-containing lipoproteins.…”