| Summary: | Macrophages and oxidized LDL (oxLDL) both play key roles in the pathogenesis of atherogenesis. This thesis is focused on the effects of oxLDL on macrophage cytokine secretion and macrophage survival. The first project defines some of the mechanisms by which oxLDL increases secretion of vascular endothelial growth factor (VEGF), a pro-inflammatory growth factor known to be involved in atherogenesis. We show that both the protein and lipid components of oxLDL contribute to induction of VEGF secretion. Disruption of the genes for CD36, SR-A, or LOX-1 scavenger receptors had no effect. The atypical protein kinase c (PKCζ) was activated by oxLDL, and this activation was essential for the induction of VEGF.
OxLDL could be atherogenic through increasing macrophage number within the plaque. Our group has previously shown that oxLDL induces growth and inhibits apoptosis in macrophages. In the second project, we sought to determine if members of the scavenger receptor family were required for the prosurvival effect of oxLDL in macrophages. We used mouse strains lacking different scavenger receptors and found that oxLDL-mediated survival is not dependent on CD36, SR-A, LOX-1, TLR4, its signaling partner CD14, or FcγRIIb. Significant inhibition of oxLDL uptake by a combined inactivation of CD36 and SR-A did not reduce the prosurvival effect of oxLDL.
In the third project, we sought to characterize the oxidative modification of LDL that is responsible for the prosurvival effect. We found that both protein and lipid components of oxLDL can induce growth in macrophages. This seems to be mediated by modification of amino groups in apoB or in phosphatidylethanolamine by lipid peroxidation products.
Further characterization of these oxidation products suggested that unfragmented hydroperoxide or endoperoxide-containing oxidation products of linoleic acid and arachidonic acid derivatize amino groups. When LDL or other proteins are modified in this fashion, they acquire the ability to induce pro-survival signaling pathways in macrophages. HPLC-MSMS studies showed that some of the arachidonic acid-derived lysine adducts are isolevuglandin (isoLG)-derived adducts that contain lactam and hydroxylactam rings. MSMS analysis of linoleic acid autoxidation adducts was consistent with 5 or 6 membered nitrogen-containing heterocycles derived from unfragmented oxidation products.
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