L54	Oxidized phospholipids as modulators of inflammation.
N.Leitinger
Department of Pharmacology and Cardiovascular Research Center, University of Virginia,, Charlottesville, US.

Although our knowledge about the mechanisms underlying atherosclerosis and its complications has dramatically increased, the question about the initiating factors of atherogenesis remains unsolved. Accumulating evidence suggests retention of low-density lipoprotein (LDL) particles in the subendothelial space with subsequent oxidative modification as key steps in atherogenesis. Oxidative modification initially gives rise to minimally oxidized LDL (MM-LDL), the biological activity of which primarily results from oxidation of phospholipids such as 1-palmitoyl-2-arachidonoyl-sn-3-glycero-phosphorylcholine (PAPC), yielding a series of structurally defined oxidation products (OxPAPC) that accumulate in atherosclerotic lesions. The atherogenic potential of OxPAPC has been demonstrated in cell culture studies as shown by enhanced monocyte binding to OxPAPC-stimulated endothelial cells, concomitant with induction of MCP-1 and IL-8. However, signalling pathways involved in oxidized phosphilipid (OxPL)-induced specific monocyte adhesion are not fully understood, and receptors for OxPL have not been described. Furthermore, OxPAPC has been shown to be biologically active in vivo when applied intravenously in mice. However, the hypothesis of OxPL as inducers of vascular inflammation in atherosclerosis remains elusive due to the lack of data obtained in adequate animal models. Thus, evidence for a direct contribution of OxPL to vascular inflammation in vivo has not yet been obtained. We could show that topical application of oxidized, but not native, phospholipids to murine carotid arteries in vivo induces expression of a set of atherosclerosis-related genes, including several chemokines, tissue factor (TF), interleukin-6 (IL-6), heme oxygenase 1 (HO-1) and early growth regulated gene 1 (EGR-1). Expression of the adhesion molecules E-selectin, vascular cell adhesion molecule 1 (VCAM-1) and intercellular adhesion molecule 1 (ICAM-1) was not induced. In isolated perfused carotid arteries, oxidized phospholipids triggered rolling and firm adhesion of monocytes in a P-selectin and keratinocyte-derived chemokine (KC)-dependent manner, which may account for monocyte accumulation mediated by P-selectin and KC seen on early atherosclerotic endothelium in apolipoprotein-E-deficient (ApoE-/-) mice. Using the murine air pouch model we showed that OxPAPC enhanced the adhesion of mononuclear cells to the air pouch wall, however, did not enhance accumulation of leukocytes in the pouch lumen. In contrast, LPS induced massive infiltration of monocytes as well as neutrophils into the air pouch cavity. Adhesion of transmigrating leukocytes can be seen at the luminal side of the air-pouch membrane after OxPAPC or LPS treatment. However, while LPS induced transmigration of leukocytes (monocytes and neutrophils into the cavity, OxPAPC preferentially induced mononuclear cell accumulation in the air-pouch wall. Moreover, we found that the kinetics of accumulation of inflammatory leukocytes in the air pouch wall in response to OxPAPC dramatically differs from that induced by LPS. Following LPS injection mainly PMNs adhered to the pouch wall, while after OxPAPC injection more than 90% of attached cells were monocytes. Moreover, the time course experiments showed that monocytic inflammation induced by OxPAPC reached a maximum at about 24 hours post application, resolving at about 48 hours. To investigate the mechanism by which OxPAPC selectively attracts mononuclear cells we analysed the pattern of chemokine expression in the air pouch wall. Our preliminary data show OxPAPC-induced production of several chemokines, which have been documented as chemoattractants for monocytes. The most prominent were JE/MCP-1 (CCL2), RANTES (CCL5), MCP-3, MIP-1a, MIP-1b, IP10 (CXCL10), BRAK (CXCL14) and SDF-1a (CXCL12). In addition, MIP-2, KC/GROa, but also HO-1 were upregulated. Thus, these data provide the first in vivo evidence for a contribution of oxidized phospholipids to vascular inflammation, suggesting novel targets for pharmacological intervention to combat atherosclerosis.

Copyright © 2005 S. Karger AG, Basel. Any further use of this abstract requires written permission from the publisher.