Oxidized low density lipoprotein acts on endothelial cells in culture to enhance endothelin secretion and monocyte migration

Monocyte deposition on the endothelium is the initial step in atherogenesis. Oxidized low density lipoprotein (Ox-LDL) is involved in the development of the fatty streak which progresses to the atherosclerotic lesion. Our interest focussed on the question, does the endothelium react to Ox-LDL to produce humoral substances that might influence the migration of human blood monocytes? Chemotaxis of monocytes was assessed by the modified membrane-filter technique based on the Boyden chamber principle. Exposure of porcine aorta endothelial cells (ECs) to Ox-LDL (100 mu g/ml) increased the directional migration of monocytes by 25% (p < 0.01) over that of ECs in the absence of Ox-LDL. Radioimmunoassay of the EC culture media revealed the presence of immunoreactive endothelin-1 (ir-ET-1). The endothelin converting enzyme inhibitor; phosphoramidone (10 mu M), when incubated together with ECs and Ox-LDL, suppressed the synthesis of ir-ET-1 by 53% (p < 0.05) and the migration decreased by 12% (p < 0.05). Preincubation of monocytes with the ETA receptor-selective antagonist, BQ-123 (1 mu M), followed by exposure to ECs plus Ox-LDL, lead to a decrease in their migration by 12% (p < 0.05) compared to monocytes not treated with BQ-123. These results show that Ox-LDL acts on ECs to enhance the synthesis of ir-ET-1 which in turn increases the directional migration of monocytes. Phosphoramidone decreased the synthesis of ir-ET-1 but migration was affected only modestly; monocyte ETA receptor blockade by BQ-123 also suppressed migration toward EC chemoattactants to a small extent. Both results suggest that in addition to ir-ET-1 other chemotactic factors are being released by the ECs; Ox-LDL appears to enhance their release or synthesis.