Apolipoprotein E produced by human monocyte-derived macrophages mediates cholesterol efflux that occurs in the absence of added cholesterol acceptors.
Human monocyte-derived macrophages can efflux accumulated cholesterol without exogenously added cholesterol acceptors (Kruth, H. S., Skarlatos, S. I., Gaynor, P. M., and Gamble, W. (1994) J. Biol. Chem. 269, 24511-24518). Most of the effluxed cholesterol accumulates in the medium as apolipoprotein E-discoidal lipid particles. In the current study, we determined whether and to what degree cholesterol efflux from human monocyte-macrophages depended on apolipoprotein E secretion. Unexpectedly, 2-week-old differentiated monocyte-macrophages secreted similar amounts of apolipoprotein E without or with cholesterol enrichment. Apolipoprotein E mRNA levels in these macrophages were not increased by cholesterol enrichment and were comparable with levels in HepG2 cells. Without cholesterol enrichment, monocyte-macrophages secreted lipid-poor apolipoprotein E with a density >1.21 g/ml. By contrast, cholesterol enrichment of monocyte-macrophages induced the association of apoE with phospholipid and cholesterol to form discoidal particles that floated at densities of 1.08-1.10 g/ml. An anti-apolipoprotein E monoclonal antibody added to the culture medium significantly inhibited cholesterol and phospholipid efflux from the monocyte-macrophages. This showed that apolipoprotein E was required for most of the cholesterol efflux, and that apolipoprotein E did not leave macrophages with lipid but rather associated with lipid after it was secreted. Thus, 1) apolipoprotein E was constitutively secreted by differentiated human monocyte-macrophages, 2) apolipoprotein E only formed discoidal particles following macrophage cholesterol enrichment, 3) apolipoprotein E was necessary for cholesterol efflux to occur in the absence of added cholesterol acceptors and, in addition 4) the level of macrophage unesterified cholesterol was not rate-limiting for this cholesterol efflux, and 5) net phospholipid synthesis occurred in macrophages secondary to apoE-mediated loss of macrophage phospholipid. In conclusion, apolipoprotein E functions in an autocrine pathway that mediates cholesterol efflux from human monocyte-derived macrophages.
Zhang
W Y
WY
Section of Experimental Atherosclerosis, NHLBI, National Institutes of Health, Bethesda, Maryland 20892, USA.
Gaynor
P M
PM
Kruth
H S
HS
D001057:Apolipoproteins E / Q000096:biosynthesis* / Q000235:genetics; D015153:Blotting, Western; D002499:Centrifugation, Density Gradient; D002784:Cholesterol / Q000378:metabolism*; D006801:Humans; D008077:Lipoproteins, LDL / Q000378:metabolism; D008264:Macrophages / Q000378:metabolism*; D009000:Monocytes / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism
D001057:Apolipoproteins E; D008077:Lipoproteins, LDL; D010743:Phospholipids; D012333:RNA, Messenger; D002784:Cholesterol