Macrophage derived foam cells are important elements of atherosclerotic plaque and several well-defined mechanisms exist by which various forms of lipoproteins enter macrophages resulting in foam cell development. These mechanisms are presumed to be of pathophysiologic importance for atherogenesis, but none have been established as causative factor(s). Details of other mechanisms of lipoprotein uptake into macrophages and of how they effect macrophage cholesterol metabolism are still needed. Our hypothesis is that clearance of lipoprotein immune complexes via one or more types of Fc-gamma receptors (Fc-gammaR) is a critical event in foam cell development and atherosclerosis. Evidence to support this hypothesis is: (1) that autoantibodies specific to epitopes of oxidized LDL exist in human atherosclerotic lesions; (2) that the presence of autoantibodies to lipoproteins such as oxidized LDL correlated with progression of atherosclerosis in humans; (3) that treatment of human macrophages with LDL immune complexes (LDL-IC) causes foam cell development via Fc-gammaR in vitro; and (4) that each type of Fc-gammaR (Fc-gammaRI, II and III) exists in macrophages of human lesions. We are proposing two related approaches to determine the importance of LDL-IC and Fc-gammaRs in atherosclerosis. Both approaches are based on the use of specific reagents that target Fc-gammaRs. With the first approach, we will study peripheral blood monocytes and plaque macrophages from patients with clinical atherosclerosis to determine if a relationship exists between levels of expression of particular Fc-gammaRs and extent of disease. With the second approach, we will determine which of the different pathways of lipoprotein uptake (e.g., LDL receptors, scavenger receptors, and Fc-gammaRs) are potentially most important in macrophage foam cell development. LDL degradation and cellular sterol mass will be determined in macrophage stimulated with LDL-IC in the context of each specific type of Fc-gammaR, and compared to controls treated with native LDL or modified lipoproteins. Several elements of the cholesterol ester cycle such as the activities and steady state mRNA levels of particular enzymes, as well as the expression and mRNA levels of LDL receptors or scavenger receptors will also be studied. These novel studies may identify new risk factors for atherosclerosis, and will advance our understanding of the regulation of cholesterol homeostasis in human macrophages.
