Oxidized phospholipids activate both platelet-activating factor receptors and peroxisome proliferator-activated receptors.

Atherosclerosis leads to about 640,000 deaths every year in the United States and is the leading cause of death in this country. Activation of monocytes by oxidized low-density lipoprotein (LDL) is a critical step in atherosclerosis. Oxidized LDL phospholipids are able to activate the platelet-activating factor (PAF) receptor. This receptor plays a key role in the activation of physiological leukocyte responses and may play a role in the development of atherosclerosis. Oxidized LDL phospholipids are also able to induce monocyte chemotactic protein-1 (MCP-1), another molecule with a critical role in both physiological inflammation and atherosclerosis. This dissertation examines the mechanisms whereby oxidized LDL phospholipids are able to induce responses in monocytes. The first study determined the structural characteristics of the oxidized LDL phospholipids that activate the PAF receptor. These phospholipids were demonstrated to be phosphatidylcholine (PC) that have sn-1 ether bonds, an unexpected result given the relative scarcity of alkylacyl PC in the LDL particle. Several species of oxidized alkylacyl PC were shown to be potent ligands of the PAF receptor. One of these PC was an alkylacyl PC with an eight carbon sn-2 acyl chain modified with an ?-aldehydic residue, 1-O-hexadecyl-2-(8-oxooctanoyl)- sn-glycero-3-phosphocholine. This oxidized PC induced PAF receptor activation at nanomolar concentrations and was as potent as several previously identified PAF analogs. The second study examined the mechanism whereby oxidized LDL phospholipids can induce MCP-1 synthesis by monocytes adherent to an alpha-ICAM-3 antibody surface. On this surface oxidized LDL phospholipids, but not PAF, stimulate MCP-1 secretion. The activation of MCP-1 secretion from monocytes was demonstrated to be independent of the PAF receptor. In the third study, the mechanism of oxidized phospholipid induction of MCP-1 secretion was further examined. Peroxisome proliferator-activated receptors (PPAR) have recently been implicated in the regulation of expression of several proteins thought to be important to the progression of atherosclerosis. Adhesion to the alpha-ICAM-3 surface by monocytes was demonstrated to induce the expression of PPAR-gamma;. PPAR agonists were demonstrated to activate MCP-1 secretion and oxidized phospholipids were themselves demonstrated to be PPAR agonists.