Effects of enrichment of fibroblasts with unesterified cholesterol on the efflux of cellular lipids to apolipoprotein A-I.
This study elucidates the factors underlying the enhancement in efflux of human fibroblast unesterified cholesterol and phospholipid (PL) by lipid-free apolipoprotein (apo) A-I that is induced by cholesterol enrichment of the cells. Doubling the unesterified cholesterol content of the plasma membrane by incubation for 24 h with low density lipoprotein and lipid/cholesterol dispersions increases the pools of PL and cholesterol available for removal by apoA-I from about 0.8-5%; the initial rates of mass release of cholesterol and PL are both increased about 6-fold. Expression of the ATP binding cassette transporter A1 (ABCA1) is critical for this increased efflux of lipids, and cholesterol loading of the fibroblasts over 24 h increases ABCA1 mRNA about 12-fold. The presence of more ABCA1 and cholesterol in the plasma membrane results in a 2-fold increase in the level of specific binding of apoA-I to the cells with no change in binding affinity. Characterization of the species released from either control or cholesterol-enriched cells indicates that the plasma membrane domains from which lipids are removed are cholesterol-enriched with respect to the average plasma membrane composition. Cholesterol enrichment of fibroblasts also affects PL synthesis, and this leads to enhanced release of phosphatidylcholine (PC) relative to sphingomyelin (SM); the ratios of PC to SM solubilized from control and cholesterol-enriched fibroblasts are approximately 2/1 and 5/1, respectively. Biosynthesis of PC is critical for this preferential release of PC and the enhanced cholesterol efflux because inhibition of PC synthesis by choline depletion reduces cholesterol efflux from cholesterol-enriched cells. Overall, it is clear that enrichment of fibroblasts with unesterified cholesterol enhances efflux of cholesterol and PL to apoA-I because of three effects, 1) increased PC biosynthesis, 2) increased PC transport via ABCA1, and 3) increased cholesterol in the plasma membrane.
Gillotte-Taylor
Kristin
K
Department of Biochemistry, MCP-Hahnemann University, Philadelphia, Pennsylvania 19129, USA.
Nickel
Margaret
M
Johnson
William J
WJ
Francone
Omar L
OL
Holvoet
Paul
P
Lund-Katz
Sissel
S
Rothblat
George H
GH
Phillips
Michael C
MC
D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters / Q000378:metabolism; D016632:Apolipoprotein A-I / Q000378:metabolism*; D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism; D002784:Cholesterol / Q000494:pharmacology*; D002794:Choline / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005347:Fibroblasts / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D050356:Lipid Metabolism; D008956:Models, Chemical; D010743:Phospholipids / Q000378:metabolism; D011485:Protein Binding; D012333:RNA, Messenger / Q000378:metabolism; D013997:Time Factors; D015854:Up-Regulation
C577836:ABCA1 protein, human; D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters; D016632:Apolipoprotein A-I; D010743:Phospholipids; D012333:RNA, Messenger; D002784:Cholesterol; D000242:Cyclic AMP; D002794:Choline