Use Of Animal Models To Study Lipid Genes That Modulate

Heart, Lung, And Blood Institute

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Abstract

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. It's role in modulating atherogenic risk is not fully understood. We have recently demonstrated that HL macrophage expression significantly contributes to atherogenesis in apoE-KO and LCAT-Tg mice. To further evaluate the role of HL, especially the ligand-binding function of HL in atherosclerosis we have generated transgenic mice that overexpress a catalytically inactive HL-145G enzyme in C57Bl/6 mice. These transgenic mice have been crossed with apoE-KO x HL-KO mice in the same genetic backround. Expression of HL-145G in apoE-KO x HL-KO mice markedly reduces the plasma total cholesterol, nonHDL-C and apoB levels and reduces proximal aortic atherosclerosis by approximately 40%. These findings demonstrate that even in the absence of lipolysis, catalytically inactive HL facilitates the plasma clearance of the apoB-containing lipoproteins and establish an important in vivo physiological function for the ligand-binding function of HL in lipoprotein metabolism and atherosclerosis in apoE-KO mice. ABCA1: Although the role of ABCA1 as the key transporter responsible for macrophage cholesterol efflux has been well studied, the function of this transporter in hepatic cholesterol transport is poorly understood. To evaluate this question we selectively expressed a human ABCA1-GFP fusion protein in the liver of C57Bl/6 mice by infusing recombinant adenovirus. After adenovirus infusion, primary hepatocytes isolated from mice expressing ABCA1-GFP exhibited enhanced cholesetrol efflux to apoA-I acceptors. Transient hepatic ABCA1 overexpression initially resulted in reduction of hepatic cholesterol followed accumulation of liver cholesterol due to compensatory upregulation of HMGCoA reductase and SR-BI. In addition, their plasma total cholesterol, HDL-C and apoA-I levels as well as nonHDL-C levels were enhanced in these mice. These findings provide in vivo evidence that ABCA1 expression in liver modulates the plasma concentrations of both HDL and nonHDL lipoproteins as well as the hepatic cholesterol concentration establishing an important role for hepatic ABCA1. ABCA1: The identification of ABCA1 as the key transporter responsible for cholesterol efflux has led to considerable interest in the development of therapeutic agents that will enhance ABCA1 expression and modulate atherogenic risk. However, the atheroprotective role of ABCA1 remains unclear. To define the role of ABCA1 in modulating plasma HDL and apoB-containing lipoprotein metabolism and atherogenesis, we crossed hABCA1-Tg with LDLR-/- mice. Compared to LDLR-/- on a chow diet (RCD), hABCA1-Tg x LDLR-/- mice had increased plasma levels of cholesterol, phospholipids, free cholesterol, cholesterol esters, nonHDL-C, apoB, and apoE but no differences in HDL-C and apoA-I. FPLC and native agarose gel electrophoresis confirmed the increased plasma levels of nonHDL-C. In addition, hepatic cholesterol levels were increased in ABCA1-Tg x LDLR-/- mice resulting in compensatory downregulation of HMGCoA reductase, SR-BI as well as enhanced biliary cholesetrol excretion in these animals. Analysis of aortic atherosclerosis on either a regular chow diet or Western diet reveals markedly enhanced aortic lesion formation in ABCA1-Tg x LDLR-/- compared to LDLR-/- mice. These findings indicate that ABCA1 modulates the metabolism of apoB-containing lipoproteins as well as of HDL, the LDLr may play a major role in facilitating ABCA1-mediated changes in nonHDL-C levels and ABCA1 overexpression worsens the proatherogenic profile and enhance atherosclerosis in LDLr-/- mice.