Overexpression of Hepatic Lipase in Transgenic Mice Decreases Apolipoprotein B-containing and High Density Lipoproteins: EVIDENCE THAT HEPATIC LIPASE ACTS AS A LIGAND FOR LIPOPROTEIN UPTAKE

Helén L Dichek,Walter Brecht,Jianglin Fan,Zhong-Sheng Ji,Sally P.A Mccormick,Hassibullah Akeefe,Lorianna Conzo,David A Sanan,Karl H Weisgraber,Stephen G Young,John M Taylor,Robert W Mahley

doi:10.1074/jbc.273.4.1896

Abstract

To determine the mechanisms by which human hepatic lipase (HL) contributes to the metabolism of apolipoprotein (apo) B-containing lipoproteins and high density lipoproteins (HDL) in vivo, we developed and characterized HL transgenic mice. HL was localized by immunohistochemistry to the liver and to the adrenal cortex. In hemizygous (hHLTg+/0) and homozygous (hHLTg+/+) mice, postheparin plasma HL activity increased by 25- and 50-fold and plasma cholesterol levels decreased by 80% and 85%, respectively. In mice fed a high fat, high cholesterol diet to increase endogenous apoB-containing lipoproteins, plasma cholesterol decreased 33% (hHLTg+/0) and 75% (hHLTg+/+). Both apoB-containing remnant lipoproteins and HDL were reduced. To extend this observation, the HL transgene was expressed in human apoB transgenic (huBTg) and apoE-deficient (apoE-/-) mice, both of which have high plasma levels of apoB-containing lipoproteins. (Note that the huBTg mice that were used in these studies were all hemizygous for the human apoB gene.) In both the huBTg,hHLTg+/0 mice and the apoE-/-,hHLTg+/0 mice, plasma cholesterol decreased by 50%. This decrease was reflected in both the apoB-containing and the HDL fractions. To determine if HL catalytic activity is required for these decreases, we expressed catalytically inactive HL (HL-CAT) in apoE-/- mice. The postheparin plasma HL activities were similar in the apoE-/- and the apoE-/-,HL-CAT+/0 mice, reflecting the activity of the endogenous mouse HL and confirming that the HL-CAT was catalytically inactive. However, the postheparin plasma HL activity was 20-fold higher in the apoE-/-,hHLTg+/0 mice, indicating expression of the active human HL. Immunoblotting demonstrated high levels of human HL in postheparin plasma of both apoE-/-,hHLTg+/0 and apoE-/-,HL-CAT+/0 mice. Plasma cholesterol and apoB-containing lipoprotein levels were approximately 60% lower in apoE-/-,HL-CAT+/0 mice than in apoE-/- mice. However, the HDL were only minimally reduced. Thus, the catalytic activity of HL is critical for its effects on HDL but not for its effects on apoB-containing lipoproteins. These results provide evidence that HL can act as a ligand to remove apoB-containing lipoproteins from plasma.

Highlights

To determine the mechanisms by which human hepatic lipase (HL) contributes to the metabolism of apolipoprotein B-containing lipoproteins and high density lipoproteins (HDL) in vivo, we developed and characterized HL transgenic mice
The HL transgene was expressed in human apoB transgenic and apoEdeficient mice, both of which have high plasma levels of apoB-containing lipoproteins. (Note that the huBTg mice that were used in these studies were all hemizygous for the human apoB gene.) In both the huBTg,hHLTg؉/0 mice and the apoE؊/؊,hHLTg؉/0 mice, plasma cholesterol decreased by 50%
To establish the mechanisms by which HL contributes to the metabolism of apoB-containing lipoproteins and HDL in vivo, we developed and characterized HL transgenic mice

Summary

Introduction

To determine the mechanisms by which human hepatic lipase (HL) contributes to the metabolism of apolipoprotein (apo) B-containing lipoproteins and high density lipoproteins (HDL) in vivo, we developed and characterized HL transgenic mice. Both apoBcontaining remnant lipoproteins and HDL were reduced To extend this observation, the HL transgene was expressed in human apoB transgenic (huBTg) and apoEdeficient (apoE؊/؊) mice, both of which have high plasma levels of apoB-containing lipoproteins. A potential role for HL in the processing of apolipoprotein (apo) B-containing lipoproteins (very low density lipoproteins (VLDL), IDL, and low density lipoproteins (LDL)) was suggested by significantly increased levels of apoB-containing lipoproteins resulting from the infusion of HL antibodies into rats or cynomolgus monkeys (9 –11) Consistent with these roles, HL-deficient patients are hypercholesterolemic and hypertriglyceridemic and accumulate ␤-migrating very low density lipoprotein (␤-VLDL) remnants, triglyceride-rich LDL, and HDL [27,28,29,30,31,32,33]

Methods

Results

Conclusion