Monolayers of mouse peritoneal macrophages were shown to synthesize and secrete a protein that resembles apoprotein E (apoE), a normal constituent of plasma lipoproteins. Synthesis and secretion were studied by incubation of macrophages with L[35S]methionine and analysis of the 35S-labeled proteins secreted into the culture medium. The 35S-labeled protein resembling apoE showed the following properties; (i) it floated in the ultracentrifuge at a density <1.215 g/ml, indicating that it was associated with lipid; (ii) by NaDodSO4/polyacrylamide gel electrophoresis, its M(r) of 35,000 was identical to that of authentic apoE obtained from mouse plasma very low density lipoprotein; (iii) its isoelectric point of 5.4 was the same as that of authentic mouse apoE; (iv) it comigrated with authentic mouse apoE after two-dimensional isoelectric focusing/NaDodSO4/polyacrylamide gel electrophoresis; and (v) it was quantitatively precipitated by a monospecific antibody directed against rat apoE. Synthesis and secretion of the apoE-like protein was stimulated 3- to 8-fold when the macrophages were loaded with cholesterol by incubation with either acetylated low density lipoprotein (acetyl-LDL) or β-migrating very low density lipoprotein from cholesterol-fed rabbits. When the cells were incubated with acetyl-LDL, the apoE-like protein composed ≃2% of the total 35S-labeled protein synthesized by the cells and ≃10% of the total 35S-labeled protein secreted into the medium. The current findings suggest a role for apoE in the plasma transport of cholesterol excreted from cholesterol-loaded macrophages.
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