HETEROGENEITY OF CELLULAR CHOLESTERYL ESTER ACCUMULATION BY HUMAN MONOCYTE-DERIVED MACROPHAGES

We have studied cholesteryl ester accumulation in human monocyte-deriv ed macrophages, which together with smooth muscle cells, represent the major cell types that accumulate cholesterol in atherosclerotic lesio ns. Monocyte-derived macrophages were incubated with either acetylated low density lipoprotein (AcLDL) or non-lipoprotein cholesterol and th e question as to whether all of the cells, or specific cell subpopulat ions could accumulate cholesteryl ester was examined. We stained chole steryl ester in monocyte-macrophages with the fluorescent probe filipi n. Cholesteryl ester accumulated as lipid droplets that were widely di spersed in the cell cytoplasm. Interestingly, no more than 65% of mono cytemacrophages accumulated cholesteryl ester during the 1st day of in cubation with non-lipoprotein cholesterol. By 2 days of incubation, gr eater than 90% of cells displayed cholesteryl ester deposition. The ch olesteryl ester which accumulated during the 2nd day of incubation was derived from unesterified cholesterol that had accumulated during the 1st day of incubation. This finding was substantiated by the followin g: (1) chemical measurements showed that the total cholesterol content of monocyte-macrophages did not increase further after the 1st day of incubation, and (2) all monocyte-macrophages had accumulated fluoresc ent tagged cholesterol during the 1st day of incubation. In contrast t o the results obtained with non-lipoprotein cholesterol, more than 90% of monocyte-macrophages incubated with AcLDL for 1 day accumulated ch olesteryl ester in two experiments. However, less than 62% of monocyte -macrophages accumulated cholesteryl ester in two other experiments, t hereby resembling results obtained with non-lipoprotein cholesterol. A gain, the lack of cholesteryl ester accumulation with AcLDL was not du e to a lack of uptake of AcLDL, as greater than 90% of monocyte-macrop hages accumulated fluorescent tagged AcLDL. The observed heterogeneity in cholesterol esterification among human monocyte-macrophages sugges ts,that functional subpopulations of these cells may exist with respec t to cholesterol processing. However, heterogeneity in cholesteryl est er accumulation did not seem to correlate with expression of HLA-DR an tigen, a marker of immunological activation of macrophages. Other sour ces of heterogeneity most likely result from inter-cellular variation at one or more levels of regulation of the cholesterol trafficking and esterification process.