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

Abstract

This article is available online at http://www.jlr.org HL is a 65 kDa glycoprotein that is synthesized and secreted primarily from parenchymal cells of the liver ( 1 ). Once secreted, HL is bound, via heparan sulfate proteoglycans (HSPG), to the surfaces of hepatic sinusoids, the external surfaces of hepatocyte microvilli in the space of Disse and in interhepatocyte spaces ( 2 ). As a member of the triglyceride lipase gene family that includes LPL and endothelial lipase (EL), HL plays a central role in lipoprotein metabolism and atherosclerosis. Like LPL and EL, HL has a number of functional domains including the serineaspartic acid-histidine catalytic triad, a lipid-binding surface loop shielding the catalytic pocket, and heparin binding domains ( 3, 4 ). Two major functions have been ascribed to HL, namely i ) a hydrolase that catalyzes hydrolysis of triacylglycerol (TAG) and phospholipids (PLs) present in circulating lipoproteins, and ii ) a ligand that facilitates binding and uptake of lipoproteins ( 5, 6 ). Both functions of HL contribute to the catabolism and clearance of lipids and lipoproteins from the circulation.In humans, HL defi ciency is associated with increased plasma concentrations of HDL as well as TAG-rich apolipoprotein (apo)B-containing lipoproteins, and also an increased prevalence of premature atherosclerosis ( 7-11 ). In mice, HL-defi ciency results in mild dyslipidemia as compared with wild-type littermates ( 12, 13 ). Studies with genetically modifi ed mice, either through overexpression or inactivation of the human HL gene ( LIPC ), have also provided evidence for HL action in apoB-containing lipoprotein metabolism. Introducing HL defi ciency into apoE-null or LDL receptor (

A Role for Hepatic Leptin Signaling in Lipid Metabolism via Altered Very Low Density Lipoprotein Composition and Liver Lipase Activity in Mice

“…This can result in more triglyceride incorporation into each VLDL particle, 17,29 leading to enlarged, more triglyceride-rich VLDL particles. 29 Intriguingly, overexpression of HL in a rat liver cell line resulted in secretion of triglyceridepoor VLDL, 30 and patients with HL deficiency have been shown to have triglyceride-rich lipoproteins that are also larger in size. 21 Therefore, although we cannot rule out the involvement of other non-LPL lipases in the liver, decreased HL activity in Lepr flox/flox AlbCreþ mice likely contributes to altered lipid loading, leading to enlarged, triglyceride-rich VLDL particles.…”

Lipidomic Analyses of Female Mice Lacking Hepatic Lipase and Endothelial Lipase Indicate Selective Modulation of Plasma Lipid Species

“…It is possible that EL might have an intracellular function associated with providing select fatty acids for the esterification of cholesterol, but it still remains to be determined if EL is catalytically active within cells. On the other hand, HL is known to become catalytically active early within the secretory pathway of hepatocytes [24, 25] and it appears to play a role in very low-density lipoprotein assembly [26, 27]. Thus, it is also possible that in the absence of EL, intracellular HL might utilize species of fatty acids that are not made available for CE.…”

Expression of human hepatic lipase negatively impacts apolipoprotein A-I production in primary hepatocytes from Lipc-null mice

“…It is known that HL can reduce concentrations of apoB-containing lipoproteins and HDL in the plasma. Our recent work with transfected McA-RH7777 cells has shown that expression of hHL had an intracellular, but non-catalytic, role in reducing the lumenal lipid droplet and impairing secretion of lipid-rich VLDL particles [20] . The present study uncovered an intracellular role of hHL in the production and/or secretion of liver derived apoA-I/HDL.…”

“…Lipolytic activity of HL has been detected within the ER/Golgi secretory pathway in transfected Chinese hamster ovary cells (CHO) and rat primary hepatocytes [18] , [19] . We documented that expression of hHLmt resulted in decreased levels of secreted apoA-I from mouse primary hepatocytes [17] , and that expression of hHL in McA-RH7777 cells, either in the catalytically-active or -inactive form (in which Ser-145 at the catalytic site of the enzyme was substituted with Gly), impaired the secretion of TAG-rich very low density lipoproteins (VLDL) [20] . Recently, Erickson et al [21] also showed that endoplasmic reticulum-localized hepatic lipase in McA-RH7777 cells decreased TAG storage and VLDL secretion.…”

“…In McA-RH7777 cells, TAG utilized for VLDL maturation is present in the microsomal lumen in the form of lipid droplets [22] . Expression of hHL, independent of its catalytic activity, diminished the newly synthesized TAG associated with the lumenal lipid droplets [20] . Thus, expression of the hHL protein within the microsomal lumen appears to negatively affect formation of the lumenal TAG pool utilized for VLDL assembly/secretion.…”