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Journal of Lipid Research, Vol. 45, 645-652, April 2004
Copyright © 2004 by American Society for Biochemistry and Molecular Biology

Potential involvement of dissociated apoA-I in the ABCA1-dependent cellular lipid release by HDL

Kei-ichiro Okuhira^*,, Maki Tsujita^*, Yoshio Yamauchi^1,*, Sumiko Abe-Dohmae^*, Koichi Kato, Tetsurou Handa and Shinji Yokoyama^2,*

^* Biochemistry, Cell Biology and Metabolism, Nagoya City University Graduate School of Medical Sciences, Nagoya 467-8601, Japan
Biocolloidal and Biointerfacial Science, Kyoto University Graduate School of Pharmaceutical Science, Kyoto 606-8501, Japan
Life Science Department, Nagoya City University Graduate School of Natural Sciences, Nagoya 467-8501, Japan

² To whom correspondence should be addressed. e-mail: syokoyam{at}med.nagoya-cu.ac.jp

Helical apolipoproteins of high density lipoprotein (HDL) remove phospholipid and cholesterol from cells and generate HDL particles being mediated by ATP binding cassette transporter A1 (ABCA1). In murine macrophage cell line RAW264 cells, cAMP induced expression of ABCA1, release of cellular phospholipid and cholesterol by apolipoprotein A-I (apoA-I), and reversible binding of apoA-I to the cell. The apoA-I-dependent lipid release was directly proportional to the cAMP-induced binding of apoA-I, and was inhibited 70% by a monoclonal antibody selective to lipid-free apoA-I, 725-1E2. In contrast, apparent cellular cholesterol release to HDL was substantial even without ABCA1 induction, and it was increased only by 27% after the cAMP treatment. The antibody inhibited this increment by 70%. Lipid-free apoA-II liberated apoA-I from HDL by displacement and thereby markedly expanded the cAMP-induced part of the cholesterol release to the HDL-containing medium, and the antibody inhibited this part also by 70%. Binding experiments of the double-labeled reconstituted HDL showed that cAMP induced reversible binding of apoA-I but not the association of cholesteryl ester with the cells. The effect of the antibody on the cellular cholesterol release to the reconstituted HDL was similar to that of the HDL-mediated release.

The data implicated that the ABCA1-dependent cholesterol release to HDL is mediated by apoA-I dissociated from HDL.

Abbreviations: apoA-I, apolipoprotein A-I; CO, cholesteryl oleate; DF medium, DMEM/F12 medium; PBS, phosphate buffered saline; rHDL, reconstituted HDL

Supplementary key words high density lipoprotein • apolipoprotein A-I • adenosine 5'-triphosphate binding cassette transporter A1 • cholesterol • macrophages

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