Cell density can affect cholesteryl ester accumulation in the human THP-1 macrophage

Esterified cholesterol (EC) accumulation was induced in THP-1 macrophages after exposure to acetylated LDL (acLDL), and the extent of accumulation was dependent on cell density. EC mass was 5-fold greater in cells plated at 1.0 x 10(6) cells/35 mm dish compared to cells plated at density 4.0 x 10(6) cells/dish. In addition, [14C]oleate incorporation into EC also increased with decreasing cell number, with 4-fold greater incorporation (6 h: 177 +/- 0.014 vs. 45 +/- 0.001 pmol/mg cell protein, P < 0.001; 24 h: 515 +/- 0.037 vs. 120 +/- 0.012 pmol/mg, P < 0.001) in cells plated less densely compared to cells plated at a higher density. The rate of 125I-labeled acLDL degradation was about 2-fold greater in cells plated at the lower density (105 vs. 60 ng/h per mg cell protein). Northern analysis showed a 2-fold reduction in the expression of human scavenger receptor mRNA in density plated cells, and immunoprecipitation also demonstrated a 2-fold decrease in scavenger receptor protein. Conditioned media did not differentially affect EC formation at either cell density. Fatty acid supplementation increased EC formation and the proportion of esterified sterol content only in cell plated at the higher density. The fatty acid effect was also seen when cells were exposed to beta-VLDL, which induced comparable levels of EC accumulation by non-scavenger receptor-mediated processes in densely plated cells. Foam cell formation in THP-1 macrophages may depend on cell density, which appears to affect both scavenger and non-scavenger receptor activity.