Loss of receptor-mediated lipid uptake via scavenger receptor A or CD36 pathways does not ameliorate atherosclerosis in hyperlipidemic mice.

Macrophage internalization of modified lipoproteins is thought to play a critical role in the initiation of atherogenesis. Two scavenger receptors, scavenger receptor A (SR-A) and CD36, have been centrally implicated in this lipid uptake process. Previous studies showed that these receptors mediated the majority of cholesterol ester accumulation in macrophages exposed to oxidized LDL and that mice with deletions of either receptor exhibited marked reductions in atherosclerosis. This work has contributed to an atherosclerosis paradigm: scavenger receptor-mediated oxidized lipoprotein uptake is required for foam cell formation and atherogenesis. In this study, Apoe-/- mice lacking SR-A or CD36, backcrossed into the C57BL/6 strain for 7 generations, were fed an atherogenic diet for 8 weeks. Hyperlipidemic Cd36-/-Apoe-/- and Msr1-/-Apoe-/- mice showed significant reductions in peritoneal macrophage lipid accumulation in vivo; however, in contrast with previous reports, this was associated with increased aortic sinus lesion areas. Characterization of aortic sinus lesions by electron microscopy and immunohistochemistry showed abundant macrophage foam cells, indicating that lipid uptake by intimal macrophages occurs in the absence of CD36 or SR-A. These data show that alternative lipid uptake mechanisms may contribute to macrophage cholesterol ester accumulation in vivo and suggest that the roles of SR-A and CD36 as proatherosclerotic mediators of modified LDL uptake in vivo need to be reassessed.

beta-Amyloid promotes accumulation of lipid peroxides by inhibiting CD36-mediated clearance of oxidized lipoproteins.

BACKGROUND: Recent studies suggest that hypercholesterolemia, an established risk factor for atherosclerosis, is also a risk factor for Alzheimer's disease. The myeloid scavenger receptor CD36 binds oxidized lipoproteins that accumulate with hypercholesterolemia and mediates their clearance from the circulation and peripheral tissues. Recently, we demonstrated that CD36 also binds fibrillar beta-amyloid and initiates a signaling cascade that regulates microglial recruitment and activation. As increased lipoprotein oxidation and accumulation of lipid peroxidation products have been reported in Alzheimer's disease, we investigated whether beta-amyloid altered oxidized lipoprotein clearance via CD36. METHODS: The availability of mice genetically deficient in class A (SRAI & II) and class B (CD36) scavenger receptors has facilitated studies to discriminate their individual actions. Using primary microglia and macrophages, we assessed the impact of Abeta on: (a) cholesterol ester accumulation by GC-MS and neutral lipid staining, (b) binding, uptake and degradation of 125I-labeled oxidized lipoproteins via CD36, SR-A and CD36/SR-A-independent pathways, (c) expression of SR-A and CD36. In addition, using mice with targeted deletions in essential kinases in the CD36-signaling cascade, we investigated whether Abeta-CD36 signaling altered metabolism of oxidized lipoproteins. RESULTS: In primary microglia and macrophages, Abeta inhibited binding, uptake and degradation of oxidized low density lipoprotein (oxLDL) in a dose-dependent manner. While untreated cells accumulated abundant cholesterol ester in the presence of oxLDL, cells treated with Abeta were devoid of cholesterol ester. Pretreatment of cells with Abeta did not affect subsequent degradation of oxidized lipoproteins, indicating that lysosomal accumulation of Abeta did not disrupt this degradation pathway. Using mice with targeted deletions of the scavenger receptors, we demonstrated that Abeta inhibited oxidized lipoprotein binding and its subsequent degradation via CD36, but not SRA, and this was independent of Abeta-CD36-signaling. Furthermore, Abeta treatment decreased CD36, but not SRA, mRNA and protein, thereby reducing cell surface expression of this oxLDL receptor. CONCLUSIONS: Together, these data demonstrate that in the presence of beta-amyloid, CD36-mediated clearance of oxidized lipoproteins is abrogated, which would promote the extracellular accumulation of these pro-inflammatory lipids and perpetuate lipid peroxidation.

Reduced atherosclerosis in MyD88-null mice links elevated serum cholesterol levels to activation of innate immunity signaling pathways.

Atherosclerosis, the leading cause of death in developed countries, has been linked to hypercholesterolemia for decades. More recently, atherosclerotic lesion progression has been shown to depend on persistent, chronic inflammation in the artery wall. Although several studies have implicated infectious agents in this process, the role of infection in atherosclerosis remains controversial. Because the involvement of monocytes and macrophages in the pathogenesis of atherosclerosis is well established, we investigated the possibility that macrophage innate immunity signaling pathways normally activated by pathogens might also be activated in response to hyperlipidemia. We examined atherosclerotic lesion development in uninfected, hyperlipidemic mice lacking expression of either lipopolysaccharide (LPS) receptor CD14 or myeloid differentiation protein-88 (MyD88), which transduces cell signaling events downstream of the Toll-like receptors (TLRs), as well as receptors for interleukin-1 (IL-1) and IL-18. Whereas the MyD88-deficient mice evinced a marked reduction in early atherosclerosis, mice deficient in CD14 had no decrease in early lesion development. Inactivation of the MyD88 pathway led to a reduction in atherosclerosis through a decrease in macrophage recruitment to the artery wall that was associated with reduced chemokine levels. These findings link elevated serum lipid levels to a proinflammatory signaling cascade that is also engaged by microbial pathogens.

Scavenger receptors class A-I/II and CD36 are the principal receptors responsible for the uptake of modified low density lipoprotein leading to lipid loading in macrophages.

Modification of low density lipoprotein (LDL) can result in the avid uptake of these lipoproteins via a family of macrophage transmembrane proteins referred to as scavenger receptors (SRs). The genetic inactivation of either of two SR family members, SR-A or CD36, has been shown previously to reduce oxidized LDL uptake in vitro and atherosclerotic lesions in mice. Several other SRs are reported to bind modified LDL, but their contribution to macrophage lipid accumulation is uncertain. We generated mice lacking both SR-A and CD36 to determine their combined impact on macrophage lipid uptake and to assess the contribution of other SRs to this process. We show that SR-A and CD36 account for 75-90% of degradation of LDL modified by acetylation or oxidation. Cholesteryl ester derived from modified lipoproteins fails to accumulate in macrophages taken from the double null mice, as assessed by histochemistry and gas chromatography-mass spectrometry. These results demonstrate that SR-A and CD36 are responsible for the preponderance of modified LDL uptake in macrophages and that other scavenger receptors do not compensate for their absence.

Accumulation of biglycan and perlecan, but not versican, in lesions of murine models of atherosclerosis.

Proteoglycan accumulation within the arterial intima has been implicated in lipoprotein retention and in atherosclerosis progression in humans. Two commonly studied murine models of atherosclerosis, the apolipoprotein E (apoE)-deficient (apoE-/-) mouse and the low density lipoprotein receptor-deficient (LDLR-/-) mouse, develop arterial lesions similar to those of human atherosclerosis. However, specific proteoglycan classes that accumulate in lesions of these mice and their relation to the retention of specific apolipoproteins have not been previously determined. In this report, we characterized the distribution of proteoglycans (versican, biglycan, and perlecan) and apolipoproteins (apoB, apoA-I, and apoE) in proximal aortic lesions of chow-fed apoE-/- and LDLR-/- mice at 10, 52, and 73 weeks of age. We observed that similar to the apoE-/- mice, the LDLR-/- mice develop intermediate and advanced plaques within 52 weeks of age. Perlecan and biglycan (both are proteoglycans) appeared early in lesion development with distinct expression patterns as the plaques advanced. Versican, a major proteoglycan detected in human plaques, was mostly absent in both strains. ApoA-I and apoB were detected in early through advanced lesions in regions of proteoglycan accumulation in both strains. Our results indicate that proteoglycans may contribute to the retention of lipoproteins at the earliest stage of atherosclerosis in murine models of atherosclerosis.