Class B Scavenger Receptors and Foam Cell Formation

The formation of cholesterol-loaded foam cells is a key event in atherogenesis. Foam cell formation is thought to be due to the unregulated receptor-mediated uptake of oxidized lipoproteins, but the identity of the lipoprotein receptors involved and their importance in lesion development in vivo are unclear. The class B scavenger receptors CD36 and SR-BI both bind oxidized LDL (oxLDL), are found in atherosclerotic lesions and in macrophages, and appear to be up regulated by oxLDL. The central hypothesis to be examined in this proposal is that the Class B scavenger receptors, CD36 and SR-BI, play important complementary roles in stimulating macrophage foam cell formation through their efficient uptake of regulatory oxidized cholesterol esters from oxidized lipoproteins and stimulation of lipoprotein uptake. Specific Aim 1: To assess the roles of macrophage CD36 and SR-BI in the uptake of regulatory oxidized cholesterol esters from oxidized lipoproteins. This will be accomplished by studying macrophage uptake of oxidized lipids from lipoproteins and determining i) the quantitative importance of endocytic uptake and selective lipid uptake pathways in CD36- and SR-BI-mediated internalization of oxidized cholesterol esters from oxLDL and ii) the efficiencies with which oxLDL and HDL deliver oxidized cholesterol esters to cells via CD-36 and SR-BI. Specific Aim 2: To determine the extent to which SR-BI and CD36 regulate PPAR-gamma activation and oxidized lipoprotein uptake in macrophages. This will be achieved by studying SR-BI- and CD36-mediated activation of PPAR-gamma by oxidized lipids and the consequent regulation of the two Class B scavenger receptors in cultured macrophages. The extent to which SR-BI mediates PPAR-gamma activation and CD36 up regulation through the uptake of oxidized lipids and the extent to which SR-BI itself is subject to positive feedback control by oxidized lipids will be quantitatively assessed. Specific Aim 3: To determine the influence of macrophage-specific expression of Class B scavenger receptors on the development of atherosclerotic lesions in mice. The influence of macrophage expression of class B scavenger receptors on foam cell formation and vascular lipid deposition will be studied in the absence of confounding changes in lipoproteins by i) bone marrow transplantation (BMT) of cells deficient in CD36 or SR-BI (derived from knock-out mice) into LDLR-/- mice and ii) over-expressing CD36 or SR-BI (by retroviral gene transfer) into LDLR-/- mice. The extent of atherosclerosis will be quantified by lesion size and cholesterol/cholesterol ester content.