Jt. Deng et al., INVESTIGATION OF PLASMA MEMBRANE-ASSOCIATED APOLIPOPROTEIN-E IN PRIMARY MACROPHAGES, Journal of lipid research, 38(2), 1997, pp. 217-227
Our previous studies identified the lysosome as the compartment for de
gradation of newly synthesized apoE in primary macrophages. Lysosomal
degradation of newly synthesized apoE is extensive and rapid (>50% in
60 min). In the present study we tested the hypothesis that the macrop
hage cell surface is part of the itinerary of apoE in its path to the
lysosomes. We therefore examined the existence and size of the apoE po
ol associated with the macrophage cell surface. Such a pool may not on
ly provide a mechanism of targeting apoE for lysosomal degradation, by
endocytosis, but also have important implications for the metabolism
of lipoproteins by macrophages. Treatment of macrophages with heparin
(10 mu g/ml and 5 mg/ml) and heparinase I (1 U/ml), which releases sub
stantial amounts of apoE from HepG2 cells, results in no additional re
lease of apoE from macrophages. Treatment of macrophages with xyloside
(1 mM) or GRGDTP (500 mu g/ml) does not decrease the extent of cell-a
ssociated apoE. Both immunogold labeling, followed by electron microsc
opy, and immunofluorescent labeling and light microscopy further confi
rm the lack of significant amounts of cell surface-associated apoE in
macrophages. In contrast, immunolabeled apoE is readily observed in pe
rmeabilized cells. Taken together, these data indicate the absence of
significant apoE-glycosaminoglycan interaction at the macrophage cell
surface. The lack of such an interaction is likely due to a paucity of
heparan sulfate proteoglycans on the macrophage cell surface, when co
mpared to hepatocytes. Along with our previous observations (Deng, J.,
V. Rudick, and L. Dory, 1995. J. Lipid Res. 36: 2129-2140), these res
ults suggest direct targeting of a portion of newly synthesized apoE f
rom trans-Golgi network to lysosomes for degradation, without involvin
g the plasma membrane and endocytosis.