Sphingolipid Signaling Induced by LDL Immune Complexes
Most of the sudden heart attacks and strokes are caused by vulnerable fatty lesions (atherosclerotic plaques) in vessel walls that are undetectable by clinical evaluation. In conditions such as diabetes and aging, LDL, the lipoprotein that carries "bad" cholesterol, becomes modified by oxidation and other chemical reactions. The body may recognize the oxidized LDL (oxLDL) as "foreign" and produce autoantibodies against it. These antibodies can bind to oxLDL and form immune complexes (oxLDL-IC), which are cleared by scavenging cells, the macrophages. Macrophages then transform into lipid-loaded cells (foam cells) and start secreting inflammatory products (cytokines). Cytokines released by macrophages and other activated cells act to promote plaque rupture and harmful consequences such as blood clotting. This proposal focuses on foam cell activation and survival induced by oxLDL-IC. The receptor Fc gamma Rl mediates the uptake of oxLDL-IC but the possible role of scavenger receptors in the uptake of oxLDL-IC and whether or not cross linking of two receptors trigger distinct signaling pathways required to elicit an enhanced macrophage response have not been examined. Our data demonstrate that exposure of U937 cells to oxLDL-IC led to increased cell survival, and to translocation and secretion of sphingosine kinase (SK). SK and its product sphingosine-1-phosphate (S1P) are known to be involved in cell proliferation and suppression of apoptosis (cell death). The central hypothesis of this application is that the process by which exposure of macrophages to oxLDL-IC leads to activation and transformation into foam cells involves the engagement of Fc gamma Rl and scavenger receptors, triggering sphingolipid signaling mechanisms that suppress apoptosis and result in extended release of cytokines. The following specific aims will be addressed: 1) characterize the involvement of specific macrophage receptors in the process of macrophage response to oxLDL-IC, and determine if sphingolipid components of oxLDL-IC contribute to the signaling that results in release of cytokines;and 2) characterize the activated downstream sphingolipid signaling response to oxLDL, and evaluate the effect of oxLDL-IC on SK1 activity and S1P signaling. These studies should uncover mechanisms by which oxLDL-IC suppress apoptosis of foam cells, and reveal specific targets in the signaling pathway that can have therapeutic implications for blocking cytokine release and to prevent formation of vulnerable plaques.