The Effects of Apolipoprotein F Deficiency on High Density Lipoprotein Cholesterol Metabolism in Mice

William R Lagor,Kaijin Wu,Denise Drazul-Schrader,Daniel J Rader,Sumeet A Khetarpal,Wen Lin,Sarah F Hamm-Alvarez,David W Fields,George H Rothblat,Arthi Kumaravel,Nathaniel Weintraub,Margarita De La Llera-Moya,Massimo Federici

doi:10.1371/journal.pone.0031616

Abstract

Apolipoprotein F (apoF) is 29 kilodalton secreted sialoglycoprotein that resides on the HDL and LDL fractions of human plasma. Human ApoF is also known as Lipid Transfer Inhibitor protein (LTIP) based on its ability to inhibit cholesteryl ester transfer protein (CETP)-mediated transfer events between lipoproteins. In contrast to other apolipoproteins, ApoF is predicted to lack strong amphipathic alpha helices and its true physiological function remains unknown. We previously showed that overexpression of Apolipoprotein F in mice reduced HDL cholesterol levels by 20–25% by accelerating clearance from the circulation. In order to investigate the effect of physiological levels of ApoF expression on HDL cholesterol metabolism, we generated ApoF deficient mice. Unexpectedly, deletion of ApoF had no substantial impact on plasma lipid concentrations, HDL size, lipid or protein composition. Sex-specific differences were observed in hepatic cholesterol content as well as serum cholesterol efflux capacity. Female ApoF KO mice had increased liver cholesteryl ester content relative to wild type controls on a chow diet (KO: 3.4+/−0.9 mg/dl vs. WT: 1.2+/−0.3 mg/dl, p<0.05). No differences were observed in ABCG1-mediated cholesterol efflux capacity in either sex. Interestingly, ApoB-depleted serum from male KO mice was less effective at promoting ABCA1-mediated cholesterol efflux from J774 macrophages relative to WT controls.

Highlights

Apolipoprotein F is an HDL-associated protein that bears no structural or sequence similarity to the other classical apolipoproteins [1,2]
We found that mouse ApoF is virtually exclusively liverexpressed, with message levels ranging from low to undetectable in the other whole tissues that were examined. (Figure 1A and 1B)
ApoF was detectable in the testes (Ct = 30.9) adrenals (Ct = 31.5), and ovaries (Ct = 33.5) at levels less than 0.6% of that seen in the liver. (Standard curves and Ct values for all tissues are included in Figure S1, and Table S1 and Table S2.)

Summary

Introduction

Apolipoprotein F (apo F) is an HDL-associated protein that bears no structural or sequence similarity to the other classical apolipoproteins [1,2]. This includes the absence of strong predicted amphipathic alpha helices which are essential for the lipid binding properties of other HDL-associated apolipoproteins such as apo A-I, apo A-II, apo E and the apo Cs [3,4]. The genes for Apolipoprotein A-I (ApoA-I) and ApoF have both been found in nearly every mammalian and fish species examined [15,16,17]. Likewise the CETP gene exists in fish, but is absent in mice and rats [18]

Methods

Results

Conclusion