Release of phagocytosed cholesterol by liver macrophages and spleen cells

Abstract

The release of radioactive cholesterol from spleen cells and liver macrophages isolated after the intravenous injection of labeled colloidal cholesterol suspensions was studied. The cells actively oxidized [I- 14C]glucose and synthesized various phospholipids and protein when incubated in albumin-containing media, but released very little labeled cholesterol. Addition of serum or serum lipoproteins to the medium increased the release of labeled cholesterol severalfold, whereas the addition of lipoprotein-free serum had no effect. Once serum lipoproteins were present the release of labeled cholesterol was not affected by the addition of cycloheximide, NaF or KCN, or by the addition of physiological concentrations of lysolecithin and linoleic acid. Depletion of serum unesteriued cholesterol by the action of lecithin cholesterol acyltransferase did not alter the release of ingested labeled cholesterol to the incubation medium. The studies indicate that reticuloendothelial cells do not secrete cholesterol. Yet, once the phagocytosed cholesterol has been solubilized, it equilibrates rapidly with the cholesterol of the serum lipoproteins in the medium. It is suggested that phagocytic cells dispose of ingested cholesterol by a continuous exchange of the sterol between cell membranes and soluble lipoproteins and that the rate of net sterol transfer is determined by the relative degree of saturation of intracellular and extracellular sterol binding sites on membranes and soluble lipoproteins.