Vladimir R. Babaev

Lack of macrophage fatty-acid-binding protein aP2 protects mice deficient in apolipoprotein E against atherosclerosis

The adipocyte fatty-acid–binding protein, aP2, has an important role in regulating systemic insulin resistance and lipid metabolism. Here we demonstrate that aP2 is also expressed in macrophages, has a significant role in their biological responses and contributes to the development of atherosclerosis. Apolipoprotein E (ApoE)-deficient mice also deficient for aP2 showed protection from atherosclerosis in the absence of significant differences in serum lipids or insulin sensitivity. aP2-deficient macrophages showed alterations in inflammatory cytokine production and a reduced ability to accumulate cholesterol esters when exposed to modified lipoproteins. Apoe−/− mice with Ap2+/+ adipocytes and Ap2−/− macrophages generated by bone-marrow transplantation showed a comparable reduction in atherosclerotic lesions to those with total aP2 deficiency, indicating an independent role for macrophage aP2 in atherogenesis. Through its distinct actions in adipocytes and macrophages, aP2 provides a link between features of the metabolic syndrome and could be a new therapeutic target for the prevention of atherosclerosis.

Treatment of diabetes and atherosclerosis by inhibiting fatty-acid-binding protein aP2

Adipocyte fatty-acid-binding protein, aP2 (FABP4) is expressed in adipocytes and macrophages, and integrates inflammatory and metabolic responses. Studies in aP2-deficient mice have shown that this lipid chaperone has a significant role in several aspects of metabolic syndrome, including type 2 diabetes and atherosclerosis. Here we demonstrate that an orally active small-molecule inhibitor of aP2 is an effective therapeutic agent against severe atherosclerosis and type 2 diabetes in mouse models. In macrophage and adipocyte cell lines with or without aP2, we also show the target specificity of this chemical intervention and its mechanisms of action on metabolic and inflammatory pathways. Our findings demonstrate that targeting aP2 with small-molecule inhibitors is possible and can lead to a new class of powerful therapeutic agents to prevent and treat metabolic diseases such as type 2 diabetes and atherosclerosis.

Reducing endoplasmic reticulum stress through a macrophage lipid chaperone alleviates atherosclerosis.

Macrophages show endoplasmic reticulum (ER) stress when exposed to lipotoxic signals associated with atherosclerosis, although the pathophysiological importance and the underlying mechanisms of this phenomenon remain unknown. Here we show that mitigation of ER stress with a chemical chaperone results in marked protection against lipotoxic death in macrophages and prevents macrophage fatty acid–binding protein-4 (aP2) expression. Using genetic and chemical models, we show that aP2 is the predominant regulator of lipid-induced macrophage ER stress. The absence of lipid chaperones incites an increase in the production of phospholipids rich in monounsaturated fatty acids and bioactive lipids that render macrophages resistant to lipid-induced ER stress. Furthermore, the impact of aP2 on macrophage lipid metabolism and the ER stress response is mediated by upregulation of key lipogenic enzymes by the liver X receptor. Our results demonstrate the central role for lipid chaperones in regulating ER homeostasis in macrophages in atherosclerosis and show that ER responses can be modified, genetically or chemically, to protect the organism against the deleterious effects of hyperlipidemia.

Cyclooxygenase-2 Promotes Early Atherosclerotic Lesion Formation in LDL Receptor–Deficient Mice

Background—Atherosclerosis has features of an inflammatory disease. Because cyclooxygenase (COX)-2 is expressed in atherosclerotic lesions and promotes inflammation, we tested the hypotheses that selective COX-2 inhibition would reduce early lesion formation in LDL receptor–deficient (LDLR−/−) mice and that macrophage COX-2 expression contributes to atherogenesis in LDLR−/− mice. Methods and Results—Treatment of male LDLR−/− mice fed the Western diet with rofecoxib or indomethacin for 6 weeks resulted in significant reductions in atherosclerosis in the proximal aorta (25% and 37%) and in the aorta en face (58% and 57%), respectively. Rofecoxib treatment did not inhibit platelet thromboxane production, a COX-1–mediated process, but it significantly reduced the urinary prostacyclin metabolite 2,3-dinor-6-keto-PGF1&agr;. Fetal liver cell transplantation was used to generate LDLR−/− mice null for expression of the COX-2 gene by macrophages. After 8 weeks on the Western diet, COX-2−/−→LDLR−/− mice developed significantly less (33% to 39%) atherosclerosis than control COX-2+/+→LDLR−/− mice. In both the inhibitor studies and the transplant studies, serum lipids did not differ significantly between groups. Conclusions—The present studies provide strong pharmacological and genetic evidence that COX-2 promotes early atherosclerotic lesion formation in LDLR−/− mice in vivo. These results support the potential of anti-inflammatory approaches to the prevention of atherosclerosis. (Circulation. 2002;105:1816-1823.)

Macrophage lipoprotein lipase promotes foam cell formation and atherosclerosis in vivo

Expression of lipoprotein lipase (LPL) by the macrophage has been proposed to promote foam cell formation and atherosclerosis, primarily on the basis of in vitro studies. LPL-deficient mice might provide a model for testing the role of LPL secretion by the macrophage in an in vivo system. Unfortunately, homozygous deficiency of LPL in the mouse is lethal shortly after birth. Because the fetal liver is the major site of hematopoiesis in the developing fetus, transplantation of C57BL/6 mice with LPL-/- fetal liver cells (FLCs) was used to investigate the physiologic role of macrophage LPL expression in vivo. Thirty-four female C57BL/6 mice were lethally irradiated and reconstituted with FLCs from day 14 LPL+/+, LPL+/-, and LPL-/- donors. No significant differences were detected in plasma levels of post-heparin LPL activity or in serum cholesterol or triglyceride levels between the 3 groups on either a chow diet or an atherogenic diet. After 19 weeks on the atherogenic diet, aortae were collected for quantitative analysis of the extent of aortic atherosclerosis. LPL expression was detected by immunocytochemistry and in situ hybridization in macrophages of aortic atherosclerotic lesions of LPL+/+-->C57BL/6 and LPL+/--->C57BL/6 mice, but not in LPL-/--->C57BL/6 mice, whereas myocardial cells expressed LPL in all groups. The mean aortic lesion area was reduced by 55% in LPL-/--->C57BL/6 mice compared with LPL+/+-->C57BL/6 mice and by 45% compared with LPL+/--->C57BL/6 mice, respectively. These data demonstrate in vivo that LPL expression by macrophages in the artery wall promotes foam cell formation and atherosclerosis. off

Inactivation of Macrophage Scavenger Receptor Class B Type I Promotes Atherosclerotic Lesion Development in Apolipoprotein E–Deficient Mice

Background—Scavenger receptor class B type I (SR-BI) is expressed in macrophages, where it has been proposed to facilitate cholesterol efflux. However, direct evidence that the expression of macrophage SR-BI is protective against atherosclerosis is lacking. In this study, we examined the in vivo role of macrophage SR-BI in atherosclerotic lesion development in the apolipoprotein (apo) E–deficient mouse model. Methods and Results—ApoE-deficient mice with (n=16) or without (n=15) expression of macrophage SR-BI were created by transplanting lethally irradiated apoE-deficient mice with bone marrow cells collected from SR-BI−/− apoE−/− mice or SR-BI+/+ apoE−/− mice. The recipient mice were fed a chow diet for 12 weeks after transplantation for analysis of atherosclerosis. Quantification of macrophage SR-BI mRNA by real-time reverse transcription–polymerase chain reaction indicated successful engraftment of donor bone marrow and inactivation of macrophage SR-BI in recipient mice reconstituted with SR-BI−/− apoE−/− bone marrow. There were no significant differences in plasma lipid levels, lipoprotein distributions, and HDL subpopulations between the 2 groups. Analysis of the proximal aorta demonstrated an 86% increase in mean atherosclerotic lesion area in SR-BI−/− apoE−/− → apoE−/− mice compared with SR-BI+/+ apoE−/− → apoE−/− mice (109.50±18.08 versus 58.75±9.58×103 &mgr;m2; mean±SEM, P =0.017). No difference in cholesterol efflux from SR-BI+/+ apoE−/− or SR-BI−/− apoE−/− macrophages to HDL or apoA-I discs was detected. Conclusions—Expression of macrophage SR-BI protects mice against atherosclerotic lesion development in apoE-deficient mice in vivo without influencing plasma lipids, HDL subpopulations, or cholesterol efflux. Thus, macrophage SR-BI plays an antiatherogenic role in vivo, providing a new therapeutic target for the design of strategies to prevent and treat atherosclerosis.

Adipocyte Fatty Acid–Binding Protein, aP2, Alters Late Atherosclerotic Lesion Formation in Severe Hypercholesterolemia

Objective—The adipocyte fatty acid–binding protein, aP2, has important effects on insulin resistance, lipid metabolism, and atherosclerosis. Its expression in macrophages enhances early foam cell formation and atherosclerosis in vivo. This study was designed to determine whether aP2 deficiency has a similar effect in the setting of advanced atherosclerosis and severe hypercholesterolemia. Methods and Results—Mice deficient in aP2 and apolipoprotein E (aP2−/−apoE−/− mice) and apolipoprotein E–deficient control mice (apoE−/− mice) were fed a Western diet for 14 weeks. No significant differences in fasting serum levels of cholesterol, triglycerides, or free fatty acids were found between groups for each sex. Compared with apoE−/− control mice, male and female aP2−/−apoE−/− mice had significant reductions in mean atherosclerotic lesion size in the proximal aorta, en face aorta, and innominate/right carotid artery. Feeding the Western diet in the apoE-deficient background did not cause a significant reduction in insulin sensitivity in vivo, as determined by steady-state serum glucose levels and insulin tolerance testing. Conclusions—These data demonstrate an important role for aP2 expression in the advanced stages of atherosclerotic lesion formation. Thus, aP2 provides an important physiological link between different features of the metabolic syndrome and is a potential target for therapy of atherosclerosis.

Combined Adipocyte-Macrophage Fatty Acid–Binding Protein Deficiency Improves Metabolism, Atherosclerosis, and Survival in Apolipoprotein E–Deficient Mice

Background—The adipocyte fatty acid–binding protein (FABP) aP2 is expressed by adipocytes and macrophages and modulates insulin resistance, glucose and lipid metabolism, and atherosclerosis. Insulin sensitivity is improved in obese but not in lean aP2-deficient mice. A second fatty acid–binding protein, mal1, also is expressed in adipocytes and macrophages, and mal1 deficiency produces similar effects on insulin resistance. We tested the hypothesis that combined aP2 and mal1 deficiency would produce synergistic effects on metabolism and reduce atherosclerosis in apolipoprotein E–deficient (apoE−/−) mice. Methods and Results—Male and female apoE−/− mice null for both aP2 and mal1 (3KO) and apoE−/− controls were fed a low-fat chow diet for 16 or 56 weeks. Lean 3KO mice had significantly lower serum cholesterol and triglycerides as well as improved insulin and glucose tolerance as compared with controls. Analysis of atherosclerotic lesions in the 3KO mice showed dramatic reductions in both early (20 weeks) and late-stage (60 weeks) atherosclerosis. Strikingly, survival in the 3KO mice was improved by 67% as compared with apoE−/− controls when challenged with the Western diet for 1 year. Conclusions—Combined aP2 and mal1 deficiency improved glucose and lipid metabolism, reduced atherosclerosis, and improved survival in apoE−/− mice, making these proteins important therapeutic targets for the prevention of the cardiovascular consequences of the metabolic syndrome.

Conditional Knockout of Macrophage PPARγIncreases Atherosclerosis in C57BL/6 and Low-Density Lipoprotein Receptor–Deficient Mice

Objective—Peroxisome proliferator-activated receptor gamma (PPARγ) is highly expressed in macrophage-derived foam cells of atherosclerotic lesions, and its expression may have a dramatic impact on atherosclerosis. Methods and Results—To investigate the contribution of macrophage PPARγ expression on atherogenesis in vivo, we generated macrophage-specific PPARγ knockout (MacPPARγKO) mice. C57BL/6 and low-density lipoprotein (LDL) receptor–deficient (LDLR−/−) mice were reconstituted with MacPPARγKO or wild-type marrow and challenged with an atherogenic diet. No differences were found in serum lipids between recipients reconstituted with MacPPARγKO and wild-type marrow. In contrast, both C57BL/6 and LDLR−/− mice transplanted with MacPPARγKO marrow had significantly larger atherosclerotic lesions than control recipients. In addition, MacPPARγKO→LDLR−/− mice had higher numbers of macrophages in atherosclerotic lesions compared with controls. Peritoneal macrophages isolated from the MacPPARγKO mice had decreased uptake of oxidized but not acetylated LDL and showed no changes in either cholesterol efflux or inflammatory cytokine expression. Macrophages from MacPPARγKO mice had increased levels of migration and CC chemokine receptor 2 (CCR2) expression compared with wild-type macrophages. Conclusion—Thus, macrophage PPARγ deficiency increases atherosclerosis under conditions of mild and severe hypercholesterolemia, indicating an antiatherogenic role for PPARγ, which may be caused, at least in part, by modulation of CCR2 expression and monocyte recruitment.

Reduced Atherosclerotic Lesions in Mice Deficient for Total or Macrophage-Specific Expression of Scavenger Receptor-A

Abstract—The absence of the scavenger receptor A (SR-A)-I/II has produced variable effects on atherosclerosis in different murine models. Therefore, we examined whether SR-AI/II deficiency affected atherogenesis in C57BL/6 mice, an inbred strain known to be susceptible to diet-induced atherosclerotic lesion formation, and whether the deletion of macrophage SR-AI/II expression would modulate lesion growth in C57BL/6 mice and LDL receptor (LDLR)−/− mice. SR-AI/II–deficient (SR-AI/II−/−) female and male mice on the C57BL/6 background were challenged with a butterfat diet for 30 weeks. No differences were detected in plasma lipids between SR-AI/II−/− and SR-AI/II+/+ mice, whereas both female and male SR-AI/II−/− mice had a tremendous reduction (81% to 86%) in lesion area of the proximal aorta compared with SR-AI/II+/+ mice. Next, to analyze the effect of macrophage-specific SR-AI/II deficiency in atherogenesis, female C57BL/6 mice were lethally irradiated, transplanted with SR-AI/II−/− or SR-AI/II+/+ fetal liver cells, and challenged with the butterfat diet for 16 weeks. In a separate experiment, male LDLR−/− mice were reconstituted with SR-AI/II−/− or SR-AI/II+/+ fetal liver cells and challenged with a Western diet for 10 weeks. No significant differences in plasma lipids and lipoprotein profiles were noted between the control and experimental groups in either experiment. SR-AI/II−/−→C57BL/6 mice, however, had a 60% reduction in lesion area of the proximal aorta compared with SR-AI/II+/+→C57BL/6 mice. A similar level of reduction (60%) in lesion area was noted in the proximal aorta and the entire aorta en face of SR-AI/II−/−→LDLR−/− mice compared with SR-AI/II+/+→LDLR−/− mice. These results demonstrate in vivo that SR-AI/II expression has no impact on plasma lipid levels and that macrophage SR-AI/II contributes significantly to atherosclerotic lesion formation.