Selective uptake of high-density lipoprotein-associated cholesteryl esters and high-density lipoprotein particle uptake by human monocyte-macrophages

Abstract

High-density lipoprotein (HDL) cholesteryl esters (CE) are taken up by many cells without parallel uptake of HDL apolipoproteins. This selective uptake of HDL CE was investigated in human monocyte-derived macrophages (HMM). HDL3 (d = 1.125-1.21 g/ml) was labeled in its apolipoprotein A-I moiety with 125I and in its CE moiety with [3H]cholesteryl oleyl ether. Cultured human monocyte-macrophages were incubated in the presence of doubly labeled HDL3 followed by determination of tracer uptake. HMM took up HDL3 particles as indicated by the uptake of HDL3 apolipoproteins. Uptake of HDL3-associated CE tracer was in significant excess of that due to HDL3 particle uptake indicating selective uptake of CE. Increased cell cholesterol due to preincubation with acetylated low-density lipoprotein (LDL) down-regulated selective uptake by HMM. According to several experimental approaches, selective uptake of HDL3 CE was independent from cell-secreted products, LDL receptor-mediated endocytosis or HDL3 retroendocytosis. The intracellular catabolism of HDL3 CE was investigated with HDL3 labeled in its CE moiety with [3H]cholesteryl oleate. The lysosomal inhibitor chloroquine had no effect on CE hydrolysis indicating that CE selectively taken up is hydrolyzed independently from lysosomes. In conclusion, HMM selectively take up HDL3-associated CE. The cellular mechanism of selective uptake is independent from endocytosis or retroendocytosis. Intracellularly, HDL3 CE selectively taken up are catabolized independently from lysosomes.