Class B Scavenger Receptors and Atherosclerosis

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The roles of the Class B scavenger receptors, SR-BI and CD36, in the development of atherosclerotic disease, a major cause of death, remain poorly understood. This is due in part to the varied and still little understood functions of these two pattern recognition receptors in host defense and tissue homeostasis. The role of CD36 in the initiation of macrophage foam cell formation is controversial, although CD36 has the ability to take up modified lipoproteins such as oxidized LDL. Other potentially important functions of CD36 include its role in the innate immune system and its role in inflammatory signaling. Atherosclerosis is a chronic inflammatory disease and we propose that CD36 plays a more complex role in atherosclerotic lesion formation. The structurally closely related receptor, SR-BI, plays a key role in reverse cholesterol transport to the liver and has been shown in several mouse models to be anti-atherogenic. Recent findings have also indicated additional extrahepatic roles of SR-BI in atherogenesis, including effects in macrophages. The functional basis for these effects is not known, although we have shown that SR-BI-null mice have a markedly enhanced inflammatory cytokine response and higher lethality in response to lipopolysaccharide (LPS). Furthermore, mice deficient in both SR-BI and CD36 unexpectedly exhibit much greater atherosclerotic lesion formation than mice lacking either SR-BI or CD36 only. These findings indicate that SR-BI and CD36 each have athero-protective functions, yet to be defined. The protective functions of the two receptors may be overlapping allowing at least partial compensation in mice lacking only one of the two receptors. The central hypothesis of this proposal is that SR-BI and CD36 act in a complementary or synergistic manner to protect against atherosclerosis. We propose that SR-BI and CD36 are key to the innate defense against atherosclerosis, a chronic inflammatory disease. The following 4 objectives are proposed to examine these hypotheses: Objective 1 will examine the complementary roles SR-BI and CD36 in atherosclerotic lesion formation in response to high fat diets. This will be accomplished by analyzing the extent and nature of aortic lesions in wild type, SR-BI-null, CD36-null and SR-BI-nullXCD36-null mice. Objective 2 will investigate the roles of macrophage-expressed Class B scavenger receptors in atherosclerosis. This will be accomplished through the use of bone marrow transplantation. Objective 3 will identify the complementary or synergistic roles of SR-B1 and CD36 in macrophage foam cell formation. This will be accomplished by measuring lipid accumulation in wild type and SR-BI-XCD36- DKO macrophages treated with modified LDLs. Cells will be examined for lipid related protein expression and by gene microarray analysis. Lipid uptake and efflux pathways will be compared. Objective 4 will investigate enhanced inflammatory signaling in CD36/SR-BI DKO macrophages and its effects on macrophage lipid accumulation. The impact of inflammatory signaling on modified LDL uptake in primary macrophages will be evaluated together with the hypothesis that increased plasma membrane cholesterol content in SR-BIxCD36 DKO macrophages stimulates inflammatory signaling pathways. These studies are expected to provide new understanding of the metabolic functions of CD36 and SR-BI as well as their role in atherosclerosis.