Investigation of plasma membrane-associated apolipoprotein E in primary macrophages

Our previous studies identified the lysosome as the compartment for degradation 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 macrophage 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 pool associated with the macrophage cell surface. Such a pool may not only 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 μg/ml and 5 mg/ml) and heparinase I (1 U/ml), which releases substantial amounts of apoE from HepG2 cells, results in no additional release of apoE from macrophages. Treatment of macrophages with xyloside (1 mM) or GRGDTP (500 μg/ml) does not decrease the extent of cell-associated apoE. Both immunogold labeling, followed by electron microscopy, and immunofluorescent labeling and light microscopy further confirm the lack of significant amounts of cell surface-associated apoE in macrophages. In contrast, immunolabeled apoE is readily observed in permeabilized 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 compared to hepatocytes. Along with our previous observations (Deng, J., V. Rudick, and L. Dory, 1995. J. Lipid Res. 36: 2129-2140), these results suggest direct targeting of a portion of newly synthesized apoE from trans-Golgi network to lysosomes for degradation, without involving the plasma membrane and endocytosis.