Mouse macrophages synthesize and secrete a protein resembling apolipoprotein E.
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 less than 1.215 g/ml, indicating that it was associated with lipid; (ii) by NaDodSO4/polyacrylamide gel electrophoresis, its Mr 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 beta-migrating very low density lipoprotein from cholesterol-fed rabbits. When the cells were incubated with acetyl-LDL, the apoE-like protein composed approximately 2% of the total 35S-labeled protein synthesized by the cells and approximately 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.
D000818:Animals; D001053:Apolipoproteins / Q000096:biosynthesis / Q000378:metabolism*; D001057:Apolipoproteins E; D001202:Ascitic Fluid / Q000166:cytology; D002784:Cholesterol / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D008264:Macrophages / Q000378:metabolism*; D008297:Male; D051379:Mice; D008970:Molecular Weight
D001053:Apolipoproteins; D001057:Apolipoproteins E; D002784:Cholesterol