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 β-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 β-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 Aβ 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 Aβ-CD36 signaling altered metabolism of oxidized lipoproteins.

Results

In primary microglia and macrophages, Aβ 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 Aβ were devoid of cholesterol ester. Pretreatment of cells with Aβ did not affect subsequent degradation of oxidized lipoproteins, indicating that lysosomal accumulation of Aβ did not disrupt this degradation pathway. Using mice with targeted deletions of the scavenger receptors, we demonstrated that Aβ inhibited oxidized lipoprotein binding and its subsequent degradation via CD36, but not SRA, and this was independent of Aβ-CD36-signaling. Furthermore, Aβ 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 β-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.

Increased atherosclerosis in streptozotocin-induced diabetic mice

Premature and extensive atheroscleroses involving renal, peripheral, and cardiovascular sites remain major complications of diabetes mellitus. Controversy exists as to the contribution of hyperglycemia versus elevated local or systemic concentrations of insulin to atherosclerosis risk. In this report, we developed the first murine model susceptible to both atherosclerosis and diabetes to determine which diabetogenic factors contribute to vascular disease. C57BL/6 and BALB/c mice were treated with multiple low-dose streptozotocin (STZ) or control citrate buffer and fed rodent chow or an atherogenic-promoting (Ath) diet for 12-20 wk. STZ treatment resulted in sustained hyperglycemia (250-420 mg/dl) and a modest reduction in plasma insulin levels for both strains regardless of diet. Citrate-treated C57BL/6 mice fed the Ath diet showed extensive oil red O-staining fatty streak aortic sinus lesions (20,537+/-2,957 micron2), the size of which did not differ for Ath-fed mice treated with STZ (16,836+/-2,136 micron2). In contrast, hyperglycemic BALB/c mice fed the Ath diet showed a 17-fold increase in atherosclerotic lesion area (7,922+/-2,096 micron2) as compared with citrate-treated mice fed the Ath diet (467+/-318 micron2). Correlations between lesion size and plasma glucose levels were significant for BALB/c (r = 0.741, P < 0.009), but not C57BL/6 (r = 0.314, P<0.3) mice. Lesion size correlated significantly with plasma cholesterol for C57BL/6 (r = 0.612, P<0.03) but not BALB/c (r = 0.630, P<0.1) mice. Immunohistochemistry showed that aortic sinus lesions from both strains contained macrophages, but smooth muscle cells were clearly present in lesions of BALB/c mice. In summary, we present the first small animal model showing accelerated atherosclerosis in response to hyperglycemia. Fatty streaks resembled those of human type II lesions in that both macrophages and smooth muscle cells were evident. In addition, our results support the concept that hyperglycemia as opposed to hyperinsulinemia contributes heavily to risk of atherosclerosis.

Effect of tolbutamide and glyburide on pyruvate kinase flux in isolated rat hepatocytes

The effects of two representative sulfonylureas, tolbutamide and glyburide, on pyruvate kinase (PK) flux were examined in fasted rat hepatocytes. PK flux was estimated by trapping 14C from NaH14CO3 in a 2 mM lactate pool, accounting for any incomplete trapping by parallel incubations with L-[1-14C]alanine. Glyburide (20 microM) and tolbutamide (1 mM) decreased glucose formation by 34.9% and 54.8%, respectively, from 2 mM lactate. This decrease in glucose formation was associated with a proportional decrease in pyruvate carboxylase (PCOX) flux (32.7% and 50.5%, respectively). Under these conditions, no net change in PK flux was observed. When hepatocytes were preincubated with lactate and/or sulfonylurea addition for 30 min prior to radiolabeling with NaH14CO3, the metabolic state of the cells changed markedly. Glyburide produced a 34.6% decrease in glucose formation and a 31.3% decrease in PCOX flux, but no change in PK flux. In contrast, tolbutamide decreased glucose formation by 12.5% and increased PK flux by 53.2%, but no change in PCOX flux was observed. Such an increase in PK flux may be linked to tolbutamide-mediated increases in fructose-1,6-bisphosphate (F16P) via fructose-2,6-bisphosphate (F26P). These findings demonstrate that tolbutamide and glyburide decrease hepatic glucose production through various alterations in carbohydrate metabolism, depending upon the metabolic state of the cell. In addition, F26P may play a larger role in the hypoglycemic mechanism of action of tolbutamide than glyburide, since pyruvate carboxylase accounted for most of the decrease in glucose formation observed with glyburide and because preincubation with tolbutamide resulted in an activation of PK.