Mononuclear leukocytes exposed to oxidized low density lipoprotein secrete a factor that stimulates endothelial cells to express adhesion molecules.

In animals fed a hypercholesterolemic diet, development of atherosclerosis is preceded by attachment of mononuclear leukocytes to the arterial endothelium. Early lesions begin to develop as monocytes migrate into the intima and ingest lipids. A major part of these lipids is believed to be derived from oxidatively modified low density lipoprotein (LDL). In the present study we demonstrate that human mononuclear leukocytes exposed to low concentrations of copper-oxidized LDL secrete one or several factors that stimulate the expression of intercellular adhesion molecule-1 (ICAM-1, CD54), vascular cell adhesion molecule (VCAM-1) and endothelial selectin (E-selectin-1, ELAM-1), whereas native LDL was found to be without effect. Exposure of endothelial cells to non-conditioned medium containing oxidized LDL did not influence the expression of adhesion molecules. Incubation of endothelial cells with conditioned medium from mononuclear cells grown in the presence of oxidized LDL also resulted in a three-fold increase in the binding of monocytoid U937 cells. The present findings suggest that mononuclear leukocytes exposed to oxidatively modified LDL in early atherosclerotic lesions may stimulate the recruitment of other leukocytes by secreting cytokines which induce the expression of adhesion molecules on the endothelium.