Mouse macrophages synthesize and secrete a protein resembling apolipoprotein E.

Abstract

Monolayers of mouse peritoneal macrophages 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 floated in the ultracentrifuge at a density < 1.215 g/ml, indicating that it was associated with lipid. By NaDodSO4 [sodium dodecyl sulfate]/polyacrylamide gel electrophoresis the MW of the apoE-like protein was 35,000, identical to that of authentic apoE obtained from mouse plasma very low density lipoprotein. The isoelectric point of the apoE-like protein was 5.4 also the same as that of authentic mouse apoE. The apoE-like protein comigrated with authentic mouse apoE after 2-dimensional isoelectric focusing/NaDodSO4/polyacrylamide gel electrophoresis and 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 .beta.-migrating very low density lipoprotein from cholesterol-fed rabbits. When the cells were incubated with acetyl-LDL, the apoE-like protein composed .apprx. 2% of the total 35S-labeled protein synthesized by the cells and .apprx. 10% of the total 35S-labeled protein secreted into the medium. apoE probably plays a role in the plasma transport of cholesterol excreted from cholesterol-loaded macrophages.