Hepatic Lipase: Hepatic lipase (HL) plays a major role in lipoprotein metabolism as both, a lipolytic enzyme and a ligand that facilitates the cellular uptake of lipoproteins. However, the separate effect of these two functions of HL on atherosclerosis has not been extensively evaluated. To investigate the separate contributions of the lipolytic vs ligand-binding functions of HL to plasma lipoprotein metabolism and atherosclerosis, we compared the effects of catalytically active HL (HL-WT) and catalytically inactive (HL-145G) HL in two different atherosucceptible mouse models with no endogenous expression of mouse HL (E-KO x HL-KO and LDLr-KO x HL-KO). HL-WT and HL-145G markedly reduced total cholesterol, nonHDL-C, and apoB, but only HL-WT decreased HDL-C and apoA-I plasma levels in E-KO mice. Compared to E-KO x HL-KO mice, both active and inactive HL lowered the pro-atherogenic lipoproteins by enhancing the catabolism of autologous 125I-apoB-VLDL/IDL and 125I-apoB-48 LDL. Similarly, even in the absence of the LDLr, HL-WT and HL-145G reduced TC, nonHDL-C, and apoB by enhancing the catabolism of autologous 125I-apoB-LDL in LDLr-KO mice; only HL-WT decreased HDL-C and apoA-I plasma levels. Infusion of RAP, which blocks LRP function, decreased the plasma clearance and hepatic uptake of 131I-apoB-48 LDL induced by HL-145G even in the absence of the LDL receptor. Despite their similar effects on lowering pro-atherogenic apoB-Lps, expression of HL-WT enhanced atherosclerosis by up to 50% in E-KO mice while expression of HL-145G markedly reduced aortic atherosclerosis by up to 96% in E-KO mice and by 34% in LDLr-KO mice. These data identify LRP as a major receptor pathway by which the ligand-binding function of HL alters apoB-Lp uptake in E-KO and LDLr-KO mice and delineate the separate contributions of the lipolytic vs. ligand-binding function of HL to plasma lipoprotein metabolism, identifying a protective, anti-atherogenic role of the ligand-binding function of HL in vi ABCA1: ABCA1 is the key transporter responsible for cellular cholesterol efflux. However, the function of this transporter in hepatic cholesterol transport is poorly understood. Recent studies in ABCA1 transgenic (C57Bl/6-ABCA1-Tg) and knockout (DBA/1J-ABCA1-KO) mice support the concept that liver ABCA1 is a major source of plasma HDL-C. To directly evaluate the specific contribution of liver ABCA1 to plasma total and HDL cholesterol we generated transgenic mice that overexpress human ABCA1 in liver and crossed them with ABCA1-KO mice (L-ABCA1-Tg x ABCA1-KO mice). Liver expression of ABCA1 normalized hepatic cholesterol efflux to apoA-I acceptors and increased the plasma TC, FC, CE, PL, and HDL-C concentrations by 60-80% (in male and female heterozygous L-ABCA1-Tg x ABCA1-KO mice) and by 90-110% (in male and female homozygous L-ABCA1-Tg x ABCA1-KO mice), respectively. Plasma lipoprotein analysis by FPLC and native agarose gel electrophoresis confirmed increased plasma HDL-C and alpha-HDL in L-ABCA1-Tg x ABCA1-KO mice. To evaluate the effect of hepatic ABCA1 expression on atherosclerosis L-ABCA1-Tg x ABCA1-KO mice have been placed on a pro-atherogenic diet and crossed with apoE-KO and LDLr-KO mice. These combined findings demonstrate that in the absence of ABCA1-mediated cholesterol efflux from extrahepatic tissues, liver ABCA1 raises plasma total and HDL-C levels, correcting the lipid and lipoprotein abnormalities in ABCA1-KO mice and definitively identify liver ABCA1 as the major source of HDL-C in plasma. The availability of L-ABCA1-Tg x ABCA1-KO mice will help to define the role of hepatic ABCA1 in atherosclerosis. ABCG5/G8:The individual roles of hepatic vs intestinal ABCG5 and ABCG8 in sterol transport have not yet been investigated. To determine the specific contribution of liver ABCG5/G8 to sterol transport and atherosclerosis, we generated transgenic mice that overexpress human ABCG5 and ABCG8 in the liver but not intestine (liver G5/G8-Tg) in three different genetic backgrounds: C57Bl/6, apoE-KO and LDLr-KO. Hepatic overexpression of ABCG5/G8 enhanced hepatobiliary secretion of cholesterol and plant sterols by 1.5-2 fold, increased the amount of intestinal cholesterol available for absorption and fecal excretion by up to 27% and decreased the accumulation of plant sterols in plasma by approximately 25%. However, it did not alter fractional intestinal cholesterol absorption, fecal neutral sterol excretion, hepatic cholesterol concentrations or hepatic cholesterol synthesis. Consequently, overexpression of ABCG5/G8 in only the liver had no effect on the plasma lipid profile, including cholesterol, HDL-C and nonHDL-C, or on the development of proximal aortic atherosclerosis in C57Bl/6, apoE-KO, or LDLr-KO mice. Thus, liver ABCG5/G8 facilitate the secretion of liver sterols into bile and serve as an alternative mechanism, independent of intestinal ABCG5/G8, to protect against the accumulation of dietary plant sterols in plasma. However, in the absence of changes in fractional intestinal cholesterol absorption, increased secretion of sterols into bile induced by hepatic overexpression of ABCG5/G8 was not sufficient to alter hepatic cholesterol balance, enhance cholesterol removal from the body or to alter atherogenic risk in liver G5/G8-Tg mice. These findings demonstrate that overexpression of ABCG5/G8 in the liver profoundly alters hepatic but not intestinal sterol transport, identifying distinct roles for liver and intestinal ABCG5/G8 in modulating sterol metabolism.
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