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IntroductionIn vitro and in vivo studies support the hypothesis that macrophages are key early mediators of atherogenesis (1-3) and their impaired recruitment and activation protects against lesion development (4-6). A significant amount of research also supports the hypothesis that subendothelial modified LDLs provide the initiating ligands for the macrophage (7,8), and these are recognized by scavenger receptors (9-13).Oxidized LDL is a ligand for the class A scavenger receptors type I and II (SRA-I/II), MARCO, the class B scavenger receptor, CD36, and the class D receptor, CD68. This modified lipoprotein has been considered the most important atherogenic LDL (14-16). In vitro studies have provided evidence that SRA-I/II and CD36 are the major oxidized LDL receptors mediating lipid accumulation and foam cell formation, whereas MARCO and CD68 play a more minor role (17-21). Absence of SRA-I/II in atherogenic murine models has had a variable impact on atherosclerosis (22-24). Thus, an essential contribution of scavenger receptors to the pathogenesis of atherosclerosis in vivo remains unresolved.CD36 has been shown to be highly regulated in monocytes/macrophages during differentiation (25,26) and to be present in lipid-laden macrophages in atherosclerotic lesions (27,28). This scavenger receptor is upregulated by IL-4 (25), macrophage colony stimulating factor (26), modified LDL (17, 18), cellular cholesterol content (29), and peroxisome proliferator-activated receptor-γ (PPAR-γ) ligands (30, 31). Unlike SRA I/II, CD36 is more broadly expressed (32-35) and has been shown to play a strategic role in lipoprotein and lipid metabolism (36, 37). The phenotype of CD36-null mice generated in our laboratory included increased plasma levels of cholesterol, triacylglycerol, and fatty acids and supported a major role for CD36 in fatty acid uptake and lipid metabolism in vivo (36). In transgenic mice, overexpression of CD36 in muscle enhanced fatty acid oxidation during stimulation/contraction and also had significant influence on plasma lipoprotein and fatty acid levels (37). Absence of CD36 was implicated recently in insulin resistance in the spontaneous hypertensive rat by using genetic analysis (37, 38). These studies point to an essential role for CD36 not only in uptake of lipid but in determination of cellular lipid stores.To determine if CD36 is a major macrophage scavenger receptor responsible for early lipid accumulation and foam cell formation, which can predispose animals to the development of fatty streaks and ultimately more advanced atherosclerotic lesions, we generated CD36-apo E double-null mice and evaluated aortic lesions on normal chow and Western diets. Macrophage scavenger receptors have been implicated as key players in the pathogenesis of atherosclerosis. To assess the role of the class B scavenger receptor CD36 in atherogenesis, we crossed a CD36-null strain with the atherogenic apo E-null strain and quantified lesion development. There was a 76.5% decrease in aortic tree lesion area (Western diet) a...

A null mutation in the scavenger receptor gene CD36 was created in mice by targeted homologous recombination. These mice produced no detectable CD36 protein, were viable, and bred normally. A significant decrease in binding and uptake of oxidized low density lipoprotein was observed in peritoneal macrophages of null mice as compared with those from control mice. CD36 null animals had a significant increase in fasting levels of cholesterol, nonesterified free fatty acids, and triacylglycerol. The increase in cholesterol was mainly within the high density lipoprotein fraction, while the increase in triacylglycerol was within the very low density lipoprotein fraction. Null animals had lower fasting serum glucose levels when compared with wild type controls. Uptake of 3 H-labeled oleate was significantly reduced in adipocytes from null mice. However, the decrease was limited to the low ratios of fatty acid:bovine serum albumin, suggesting that CD36 was necessary for the high affinity component of the uptake process. The data provide evidence for a functional role for CD36 in lipoprotein/fatty acid metabolism that was previously underappreciated.Scavenger receptors are integral membrane glycoproteins, distinct from the classic low density lipoprotein (LDL) 1 receptor, that mediate binding and uptake of native and modified lipoproteins by macrophages (1-8). There are at least two major classes of mammalian monocyte/macrophage scavenger receptors, SR-A and SR-B, based on molecular sequence and protein structural homology (1, 2, 9 -11). Scavenger receptors have broad ligand specificity and may have evolved from the primitive immune system as pattern recognition molecules, which are able to recognize common structural motifs on microbial surfaces (1,6,(12)(13)(14)(15)(16)(17). They also function in the recognition and clearance of damaged, senescent, or apoptotic cells before lysis, tissue damage, and inflammation can ensue (11, 18 -21) and in the modulation of cytokine release and host immune responses (14,15,22). Scavenger receptors may be important in the pathogenesis of atherosclerosis, since there is significant evidence in support of the hypothesis that uptake of oxidatively modified LDL by monocytes/macrophages is one of the key early events in lesion development (23-26).The class A receptors, which are expressed on liver sinusoidal endothelial and Kupffer cells (27)(28)(29), and monocytes/ macrophages (9, 10, 30) result from an alternative splice from a single gene (31, 32). SR-AI/II are trimeric, integral membrane glycoprotein receptors for oxidized LDL, acetylated LDL, and other anionic ligands including polyinosinic acid and maleylated albumin (5, 9, 10, 33-35). Monocytes/macrophages isolated from a null mouse carrying a mutation in the class A receptors showed partial loss in the ability to bind and internalize oxidized LDL (ϳ30%) (36), and a lack of murine SR-AI/II receptors in the context of an atherogenic environment was partially protective against the formation of atherogenic lesions, decreasing lesion...

The macrophage scavenger receptor CD36 plays an important role in binding and uptake of oxidized forms of low-density lipoprotein (LDL), foam cell formation, and lesion development during atherosclerosis. The structural basis of CD36-lipoprotein ligand recognition is an area of intense interest. In a companion article we reported the characterization of a structurally conserved family of oxidized choline glycerophospholipids (ox-PC CD36 ) that serve as novel high affinity ligands for cells stably transfected with CD36, mediating recognition of multiple oxidized forms of LDL (Podrez, E. A., Poliakov, E., Shen, Z., Zhang, R., Deng, Y., Sun, M., Finton, P., Shan, L., Gugiu, B., Fox, P. L., Hoff, H. F., Salomon, R. G., and Hazen, S. L. (July 8, 2002) J. Biol. Chem. 277, 10.1074/ jbc.M203318200). Here we use macrophages from wildtype and CD36 null mice to demonstrate that CD36 is the major receptor on macrophages mediating recognition of oxPC CD36 species when presented (؉/؊ plasma) in pure form, within PC bilayers in small unilamellar vesicles, and within liposomes generated from lipid extracts of native LDL. We also show that oxPC CD36 promote CD36-dependent recognition when present at only a few molecules per particle, resulting in macrophage binding, uptake, metabolism, cholesterol accumulation, and foam cell formation. Finally, using high performance liquid chromatography with on-line electrospray ionization tandem mass spectrometry (LC/ESI/MS/MS), we demonstrate that oxPC CD36 are generated in vivo and are enriched in atherosclerotic lesions. Collectively, our data suggest that formation of this novel family of oxidized phospholipids participates in CD36-mediated recognition of oxidized lipoproteins and foam cell formation in vivo.CD36 is a multifunctional cellular receptor with broad ligand specificity (1, 2). It is expressed in a number of cells including microvascular endothelial cells, platelets, adipocytes, striated muscles, macrophages, and some vascular smooth muscle cells (1, 2). CD36 regulates cellular adhesion and angiogenesis, serving as a receptor for thrombospondin; it also serves as a scavenger receptor in macrophages, mediating uptake of apoptotic cells and modified lipoproteins, and participates in carbohydrate and lipid metabolism, modulating insulin resistance and long chain fatty acid transport (3-8). CD36 has recently been implicated in a variety of pathologic conditions, including atherosclerosis, diabetes, and cardiomyopathy. Perhaps the most compelling data on the role of CD36 in atherosclerosis are from studies of CD36-deficient mice, which show a 70 -80% reduction in aortic lesion size (9). In vitro experiments demonstrate that macrophages from CD36-deficient mice take up different forms of oxidized LDL 1 poorly and are resistant to foam cell formation, providing a mechanism for the atheroprotection observed in CD36 null mice (9, 10). Given the potential clinical significance of this receptor, it is important to know the nature of the ligand(s) in oxidized LDL (oxLDL) that are recognize...

Abstract-Atherosclerosis is now understood to be a disease characterized by inflammation that results in a host of complications, including ischemia, acute coronary syndromes (unstable angina pectoris and myocardial infarction), and stroke. Inflammation may be caused by a response to oxidized low-density lipoproteins, chronic infection, or other factors; and markers of this process, such as C-reactive protein, may be useful to predict an increased risk of coronary heart disease. Thus, we believe that inflammatory processes may be potential targets of therapy in preventing or treating atherosclerosis and its complications.

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Vascular endothelial growth factor (VEGF) receptorFlk-1/KDR in endothelial cells is activated during vasculogenesis and angiogenesis upon ligand-receptor interaction. Activated Flk-1/KDR has been shown to recruit Src homology 2 domain-containing signaling molecules that are known to serve as links to the activation of the mitogen-activated protein (MAP) kinase signaling pathway. To define the functional significance of phosphatidylinositol (PI) 3-kinase in VEGF signaling, we have examined its role in human umbilical vein endothelial cell (HUVEC) cycle progression. We show herein that p85, the regulatory subunit of PI 3-kinase, is constitutively associated with Flk-1/KDR. The treatment of HUVECs with VEGF promoted tyrosine autophosphorylation of Flk-1/KDR and also induced phosphorylation of p85. This was followed by an increase in the PI 3-kinase activity, which was sensitive to wortmannin, a potent PI 3-kinase inhibitor. VEGF also induced a striking activation of MAP kinase in a time-dependent manner. Inhibition studies with both a dominant-negative p85 mutant and the PI 3-kinase inhibitor, wortmannin, were employed to show for the first time that VEGF-stimulated PI 3-kinase modulates MAP kinase activation and nuclear events such as transcription from c-fos promoter and entry into the synthesis (S)-phase. Our data demonstrate the importance of PI 3-kinase as a necessary signaling component of VEGF-mediated cell cycle progression.