Temporary sequestration of cholesterol and phosphatidylcholine within extracellular domains of ABCA1 during nascent HDL generation

Masato Ishigami,Kohjiro Nagao,Kazumitsu Ueda,Fumihiko Ogasawara,Hidehiko Hashimoto,Noriyuki Kioka,Yasuhisa Kimura

doi:10.1038/s41598-018-24428-6

Masato Ishigami, Kohjiro Nagao + Show 5 more

Open Access

https://doi.org/10.1038/s41598-018-24428-6

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Journal: Scientific Reports	Publication Date: Apr 18, 2018
Citations: 39	License type: open-access

Affiliation: Kyoto University

Abstract

The quality and quantity of high-density lipoprotein (HDL) in blood plasma are important for preventing coronary artery disease. ATP-binding cassette protein A1 (ABCA1) and apolipoprotein A-I (apoA-I) play essential roles in nascent HDL formation, but controversy persists regarding the mechanism by which nascent HDL is generated. In the “direct loading model”, apoA-I acquires lipids directly from ABCA1 while it is bound to the transporter. By contrast, in the “indirect model”, apoA-I acquires lipids from the specific membrane domains created by ABCA1. In this study, we found that trypsin treatment causes rapid release of phosphatidylcholine (PC) and cholesterol from BHK/ABCA1 cells, and that the time course of lipid release coincides with those of trypsin digestion of extracellular domains (ECDs) of surface ABCA1 and of release of ECD fragments into the medium. This trypsin-dependent lipid release was dependent on ABCA1 ATPase activity, and did not occur in cells that express ABCG1, which exports lipids like ABCA1 but does not have large ECDs. These results suggest that the trypsin-sensitive sites on the cell surface are the large ECDs of ABCA1, and that lipids transported by ABCA1 are temporarily sequestered within the ECDs during nascent HDL formation.

Highlights

The quality and quantity of high-density lipoprotein (HDL) in blood plasma are important for preventing coronary artery disease[1]
We found that trypsin treatment causes the rapid release of PC and cholesterol from baby hamster kidney (BHK)/ATP-binding cassette protein A1 (ABCA1) cells, suggesting that PC and cholesterol are temporarily sequestered at trypsin-sensitive sites on the surface of cells in an ATP-dependent manner
These results suggest that the trypsin-sensitive sites on the cell surface are the large extracellular domains (ECDs) of ABCA1, and that lipids transported by ABCA1 are temporarily sequestered within the ECDs during nascent HDL formation

Summary

Introduction

The quality and quantity of high-density lipoprotein (HDL) in blood plasma are important for preventing coronary artery disease[1]. ATP-binding cassette protein A1 (ABCA1) exports excess cellular cholesterol and phosphatidylcholine (PC) to lipid-free apolipoprotein A-I (apoA-I) in serum[2], thereby generating nascent HDL, a bilayer fragment consisting of 200–700 lipids wrapped by two to four molecules of apoA-I3,4 This step of nascent discoidal HDL generation is critical for HDL formation, as demonstrated by the fact that missense mutations in ABCA1 cause Tangier disease, a condition in which patients have very low or no circulating HDL5–8. In the direct loading model, apoA-I acquires lipids directly from ABCA1 while it is bound to the transporter, whereas in the indirect model, it acquires lipids from the specific membrane domains created by the phospholipid translocation activity of ABCA1 The latter model is supported by the existence of two types of apoA-I binding sites on the plasma membranes of cells expressing ABCA1, namely a high-affinity/low-capacity binding site and a low-affinity/ high-capacity binding site[20,21]. These results suggest that the trypsin-sensitive sites on the cell surface are the large ECDs of ABCA1, and that lipids transported by ABCA1 are temporarily sequestered within the ECDs during nascent HDL formation

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Conclusion