Unexpected Expression Pattern for Glycosylphosphatidylinositol-anchored HDL-binding Protein 1 (GPIHBP1) in Mouse Tissues Revealed by Positron Emission Tomography Scanning

Tove Olafsen,Stephen G Young,Brandon S.J Davies,Anne P Beigneux,Vania E Kenanova,Constance Voss,Glen Young,Koon-Pong Wong,Richard H Barnes,Yiping Tu,Michael M Weinstein,Chika Nobumori,Sung-Cheng Huang,Ira J Goldberg,André Bensadoun,Anna M Wu,Loren G Fong

doi:10.1074/jbc.m110.171041

Abstract

Glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1), a GPI-anchored endothelial cell protein, binds lipoprotein lipase (LPL) and transports it into the lumen of capillaries where it hydrolyzes triglycerides in lipoproteins. GPIHBP1 is assumed to be expressed mainly within the heart, skeletal muscle, and adipose tissue, the sites where most lipolysis occurs, but the tissue pattern of GPIHBP1 expression has never been evaluated systematically. Because GPIHBP1 is found on the luminal face of capillaries, we predicted that it would be possible to define GPIHBP1 expression patterns with radiolabeled GPIHBP1-specific antibodies and positron emission tomography (PET) scanning. In Gpihbp1(-/-) mice, GPIHBP1-specific antibodies were cleared slowly from the blood, and PET imaging showed retention of the antibodies in the blood pools (heart and great vessels). In Gpihbp1(+/+) mice, the antibodies were cleared extremely rapidly from the blood and, to our surprise, were taken up mainly by lung and liver. Immunofluorescence microscopy confirmed the presence of GPIHBP1 in the capillary endothelium of both lung and liver. In most tissues with high levels of Gpihbp1 expression, Lpl expression was also high, but the lung was an exception (very high Gpihbp1 expression and extremely low Lpl expression). Despite low Lpl transcript levels, however, LPL protein was readily detectable in the lung, suggesting that some of that LPL originates elsewhere and then is captured by GPIHBP1 in the lung. In support of this concept, lung LPL levels were significantly lower in Gpihbp1(-/-) mice than in Gpihbp1(+/+) mice. In addition, Lpl(-/-) mice expressing human LPL exclusively in muscle contained high levels of human LPL in the lung.

Highlights

The triglyceride-rich lipoproteins undergo lipolytic processing in the capillaries of peripheral tissues, mainly in heart and skeletal muscle, where the lipids are used as fuel, and in adipose tissue, where the lipids are stored [1]
Lipolysis depends on lipoprotein lipase (LPL),4 an enzyme that is synthesized and secreted at high levels by myocytes and adipocytes [1, 2]
Davies and co-workers [7] made sense of the hyperlipidemia: they showed that GPIHBP1 shuttles LPL from the interstitial spaces of tissues, where it is secreted by parenchymal cells, into the capillary lumen, the site where it needs to be to hydrolyze triglycerides in lipoproteins

Summary

Introduction

The triglyceride-rich lipoproteins (chylomicrons and very low density lipoproteins) undergo lipolytic processing in the capillaries of peripheral tissues, mainly in heart and skeletal muscle, where the lipids are used as fuel, and in adipose tissue, where the lipids are stored [1]. To assess tissue uptake at 30 min, wild-type and Gpihbp1Ϫ/Ϫ 124I-labeled rat monoclonal antibody against GPIHBP1 Uptake of radioactivity between wild-type and Gpihbp1Ϫ/Ϫ mice were observed in into tissues was corrected for decay and expressed as a percent- lungs (p ϭ 0.0103) and blood (p ϭ 0.00003); significant differage of injected dose/g of tissue (% ID/g) (mean Ϯ S.E. in cases ences were observed in liver, spleen, kidneys, heart, skeletal where groups of four mice were evaluated).

Results

Conclusion