LDL stimulates chemotaxis of human monocytes through a cyclooxygenase-dependent pathway

Monocyte migration into the vessel wall is an early step in atherogenesis. Even though a number of chemotactic factors have been identified, the regulation of the chemotactic response is not clearly understood. As the release of arachidonic acid has been implicated in monocyte chemotaxis, we studied the influence of LDL, which can supply this fatty acid to cells, on the chemotactic mobility of monocytes. Migration of human monocytic U937 cells was abolished by a 30-hour incubation in medium containing lipoprotein-depleted 10% fetal calf serum. Thereafter, human VLDL, LDL, acetyl LDL, methyl LDL, HDL, free cholesterol, linoleic acid, oleic acid, or arachidonic acid was added. At the end of varying incubation periods (0.5 to 8 hours), chemotaxis, viability, and cellular cholesterol content were measured. In the same experimental setting we also studied the effects of the pharmacological agents chloroquine, indomethacin, and acetylsalicylic acid on LDL-mediated chemotaxis. Chemotaxis was restored by LDL in a dose- and time-dependent manner starting at concentrations as low as 5 micrograms/mL and at incubations as brief as 30 minutes. The other lipoproteins tested (VLDL, HDL, acetyl LDL, and methyl LDL) as well as free cholesterol had no comparable effect on chemotaxis. Viability and total cholesterol content did not differ among the groups. Simultaneous incubation of cells with chloroquine, indomethacin, and acetylsalicylic acid reduced restitution of chemotaxis by LDL by 71%, 82%, and 68%, respectively. In contrast, the agents had only slight inhibitory effects on the chemotactic mobility of serum-fed control cells. Incubation with linoleic acid showed a 60% restoration of chemotaxis, whereas arachidonic acid stimulated chemotaxis by 140% compared with the positive control. Preincubation of LDL with the monoclonal antibody MB47 directed against LDL resulted in a significantly reduced migratory response. The data suggest a novel cyclooxygenase-dependent regulatory mechanism of chemotaxis by LDL.