CELL-DENSITY CAN AFFECT CHOLESTERYL ESTER ACCUMULATION IN THE HUMAN THP-I MACROPHAGE

Esterified cholesterol (EC) accumulation was induced in THP-1 macropha ges after exposure to acetylated LDL (acLDL), and the extent of accumu lation was dependent on cell density. EC mass was 5-fold greater in ce lls 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, [C-14]oleate incorporati on into EC also increased with decreasing cell number, with 4-fold gre ater incorporation (6 h: 177 +/- 0.014 vs. 45 +/- 0.001 pmol/mg cell p rotein, 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 high er density. The rate of I-125-labeled acLDL degradation was about P-fo ld greater in cells plated at the lower density (105 vs. 60 ng/h per m g cell protein). Northern analysis showed a 2-fold reduction in the ex pression of human scavenger receptor mRNA in densely plated cells, and immunoprecipitation also demonstrated a 2-fold decrease in scavenger receptor protein. Conditioned media did not differentially affect EC f ormation at either cell density. Fatty acid supplementation increased EC formation and the proportion of esterified sterol content only in c ells plated at the higher density The fatty acid effect was also seen when cells were exposed to P-VLDL, which induced comparable levels of EC accumulation by non-scavenger receptor-mediated processes in densel y plated cells. Foam cell formation in THP-1 macrophages may depend on cell density, which appears to affect both scavenger and non-scavenge r receptor activity.