Mouse peritoneal macrophages were cultured for 24 h in Dulbecco-Vogt medium containing 10% calf serum. This medium was replaced with Dulbecco-Vogt medium containing 1% bovine serum albumin to which all subsequent additions were made. Medium changes, accompanied by appropriate additions, were made every 48 or 72 h and the media were used for incubation of aortic smooth muscle cells, prelabeled with [3H]cholesterol. The amount of labeled cholesteryl ester in the smooth muscle cells incubated for 48 h with macrophage media which had been collected 48-144 h after addition of acetylated LDL was increased 3-4 times above that present prior to postincubation. A marked increment in cholesteryl ester mass occurred also after incubation of smooth muscle cells with macrophage media conditioned with acetylated LDL and this effect was shared by maleylated LDL, but not by other negatively charged compounds. The increase in labeled cholesteryl ester in smooth muscle cells was more pronounced with media collected at later time intervals of incubation with macrophages and was evident 8 hr after postincubation. Only the d less than 1.063 fraction of the medium enhanced cholesterol esterification in smooth muscle cells. The acetylated LDL reisolated from macrophage media at d less than 1.063 did not compete with native LDL for degradation by smooth muscle cells. No increase in degradation of 125I-labeled acetylated LDL preincubated with macrophages was observed above that of non-preincubated acetylated LDL. The macrophage medium conditioned with acetylated LDL depressed [14C]acetate incorporation into sterols in smooth muscle cells and this effect was abolished by extraction of the medium with diethyl ether. The ratio of free to total cholesterol in the macrophage media collected after incubation with acetylated LDL increased from 28-70%, and a decrease occurred after incubation with smooth muscle cells. The enhancement of cholesterol esterification could be abolished by addition of high density apolipoprotein/sphingomyelin mixture during incubation with macrophages, even though excretion of free cholesterol into the medium increased 3-fold. It is proposed that when smooth muscle cells are presented with a lipoprotein in which an increase in the free to esterified cholesterol ratio occurred, and which is not recognized by a specific receptor, the enhancement of cellular cholesterol esterification is due mostly to a surface transfer of lipoprotein-free cholesterol. The present results offer another view of the possible interactions between macrophages and smooth muscle cells. A modified lipoprotein, not recognized by smooth muscle cells, is ingested by macrophages, which leads to accumulation of esterified cholesterol. Part of the esterified cholesterol undergoes hydrolysis and is excreted back into the medium, leading to enrichment of the lipoproteins in the medium with free cholesterol. This enrichment with free cholesterol promotes cholesterol esterification in smooth muscle cells.
Acylation Animals