Macrophage apoE Research Articles

Low-Density Lipoprotein Receptor–Related Protein 1 Prevents Early Atherosclerosis by Limiting Lesional Apoptosis and Inflammatory Ly-6C high Monocytosis

Background— We previously demonstrated that macrophage low-density lipoprotein receptor (LDLR)–related protein 1 (LRP1) deficiency increases atherosclerosis despite antiatherogenic changes including decreased uptake of remnants and increased secretion of apolipoprotein E (apoE). Thus, our objective was to determine whether the atheroprotective effects of LRP1 require interaction with apoE, one of its ligands with multiple beneficial effects. Methods and Results— We examined atherosclerosis development in mice with specific deletion of macrophage LRP1 (apoE −/− MΦLRP1 −/− ) and in LDLR −/− mice reconstituted with apoE −/− MΦLRP1 −/− bone marrow. The combined absence of apoE and LRP1 promoted atherogenesis more than did macrophage apoE deletion alone in both apoE-producing LDLR −/− mice (+88%) and apoE −/− mice (+163%). The lesions of both mouse models with apoE −/− LRP1 −/− macrophages had increased macrophage content. In vitro, apoE and LRP1 additively inhibit macrophage apoptosis. Furthermore, there was excessive accumulation of apoptotic cells in lesions of both LDLR −/− mice (+110%) and apoE −/− MΦLRP1 −/− mice (+252%). The apoptotic cell accumulation was partially due to decreased efferocytosis as the ratio of free to cell-associated apoptotic nuclei was 3.5-fold higher in lesions of apoE −/− MΦLRP1 −/− versus apoE −/− mice. Lesion necrosis was also increased (6 fold) in apoE −/− MΦLRP1 −/− versus apoE −/− mice. Compared with apoE −/− mice, the spleens of apoE −/− MΦLRP1 −/− mice contained 1.6- and 2.4-fold more total and Ly6-C high monocytes. Finally, there were 3.6- and 2.4-fold increases in Ly6-C high and CC-chemokine receptor 2–positive cells in lesions of apoE −/− MΦLRP1 −/− versus apoE −/− mice, suggesting that accumulation of apoptotic cells enhances lesion development and macrophage content by promoting the recruitment of inflammatory monocytes. Conclusion— Low-density lipoprotein receptor protein 1 exerts antiatherogenic effects via pathways independent of apoE involving macrophage apoptosis and monocyte recruitment.

A novel role for c-Jun N-terminal kinase and phosphoinositide 3-kinase in the liver X receptor-mediated induction of macrophage gene expression

Liver X receptors (LXRs) are ligand-dependent transcription factors that are activated by metabolites of cholesterol, oxysterols, and a number of synthetic agonists. LXRs play potent anti-atherogenic roles in part by stimulating the efflux of cholesterol from macrophage foam cells. The LXR-induced expression of ATP-binding cassette transporter (ABC)-A1 and Apolipoprotein E (ApoE) in macrophages is essential for the stimulation of cholesterol efflux and the prevention of atherosclerotic development. Unfortunately, the signaling pathways underlying such regulation are poorly understood and were therefore investigated in human macrophages. The expression of ApoE and ABCA1 induced by synthetic or natural LXR ligands [TO901317, GW3965, and 22-(R)-hydroxycholesterol (22-(R)-HC), respectively] was attenuated by inhibitors of c-Jun N-terminal kinase (JNK) (curcumin and SP600125) and phosphoinositide 3-kinase (PI3K) (LY294002). Similar results were obtained with ABCG1 and LXR-α, two other LXR target genes. LXR agonists activated several components of the JNK pathway (SEK1, JNK and c-Jun) along with AKT, a downstream target for PI3K. In addition, dominant negative mutants of JNK and PI3K pathways inhibited the LXR-agonists-induced activity of the ABCA1 and LXR-α gene promoters in transfected cells. LXR agonists also induced the binding of activator protein-1 (AP-1), a key transcription factor family regulated by JNK, to recognition sequences present in the regulatory regions of the ApoE and ABCA1 genes. These studies reveal a novel role for JNK and PI3K/AKT signaling in the LXR-regulated expression in macrophages of several key genes implicated in atherosclerosis.

Macrophage, But Not Systemic, Apolipoprotein E Is Necessary for Macrophage Reverse Cholesterol Transport In Vivo

To assess the role of apolipoprotein (apo) E in macrophage reverse cholesterol transport (RCT) in vivo. ApoE exerts an antiatherosclerotic activity by regulating lipoprotein metabolism and promoting cell cholesterol efflux. We discriminated between macrophage and systemic apoE contribution using an assay of macrophage RCT in mice. The complete absence of apoE lead to an overall impairment of the process and, similarly, the absence of apoE exclusively in macrophages resulted in the reduction of cholesterol mobilization from macrophages to plasma, liver, and feces. Conversely, expression of apoE in macrophages is sufficient to promote normal RCT even in apoE-deficient mice. The mechanisms accounting for these results were investigated by evaluating the first step of RCT (ie, cholesterol efflux from cells). Macrophages isolated from apoE-deficient mice showed a reduced ability to release cholesterol into the culture medium, whereas the apoB-depleted plasma from apoE-deficient and healthy mice possessed a similar capacity to promote cellular lipid release from cultured macrophages. Our data demonstrate, for the first time to our knowledge, that apoE significantly contributes to macrophage RCT in vivo and that this role is fully attributable to apoE expressed in macrophages.

The role of reverse cholesterol transport in animals and humans and relationship to atherosclerosis

Reverse cholesterol transport (RCT) is a term used to describe the efflux of excess cellular cholesterol from peripheral tissues and its return to the liver for excretion in the bile and ultimately the feces. It is believed to be a critical mechanism by which HDL exert a protective effect on the development of atherosclerosis. In this paradigm, cholesterol is effluxed from arterial macrophages to extracellular HDL-based acceptors through the action of transporters such as ABCA1 and ABCG1. After efflux to HDL, cholesterol may be esterified in the plasma by the enzyme lecithin:cholesterol acyltransferase and is ultimately transported from HDL to the liver, either directly via the scavenger receptor BI or after transfer to apolipoprotein B-containing lipoproteins by the cholesteryl ester transfer protein. Methods for assessing the integrated rate of macrophage RCT in animals have provided insights into the molecular regulation of the process and suggest that the dynamic rate of macrophage RCT is more strongly associated with atherosclerosis than the steady-state plasma concentration of HDL cholesterol. Promotion of macrophage RCT is a potential therapeutic approach to preventing or regressing atherosclerotic vascular disease, but robust measures of RCT in humans will be needed in order to confidently advance RCT-promoting therapies in clinical development.

Apolipoprotein E knock-out and knock-in mice: atherosclerosis, metabolic syndrome, and beyond

Given the multiple differences between mice and men, it was once thought that mice could not be used to model atherosclerosis, principally a human disease. Apolipoprotein E-deficient (apoEKO) mice have convincingly changed this view, and the ability to model human-like plaques in these mice has provided scientists a platform to study multiple facets of atherogenesis and to explore potential therapeutic interventions. In addition to its well-established role in lipoprotein metabolism, recent observations of reduced adiposity and improved glucose homeostasis in apoEKO mice suggest that apoE may also play a key role in energy metabolism in peripheral organs, including adipose tissue. Finally, along with apoEKO mice, knockin mice expressing human apoE isoforms in place of endogenous mouse apoE have provided insights into how quantitative and qualitative genetic alterations interact with the environment in the pathogenesis of complex human diseases.

Independent protective roles for macrophage Abcg1 and Apoe in the atherosclerotic lesion development

Objective ATP-binding cassette transporter G1 (Abcg1) and apolipoprotein E (Apoe) play a role in macrophage cholesterol efflux and consequently the development of atherosclerosis. A possible interaction between Abcg1 and Apoe in cholesterol efflux was postulated, but the potential combined action of these proteins on atherosclerotic lesion formation is unclear. Methods LDL receptor knockout (KO) mice were transplanted with bone marrow from Abcg1/Apoe double KO (dKO) mice, their respective single knockouts, and wild-type (WT) controls and challenged with a high-fat/high-cholesterol diet for 6 weeks to induce atherosclerosis. Results No differences were found in serum lipid levels. The mean atherosclerotic lesion area in dKO transplanted animals (187 ± 18 × 10 3 μm 2) was 1.4-fold ( p < 0.01) increased compared to single knockouts (Abcg1 KO: 138 ± 5 × 10 3 μm 2; Apoe KO: 131 ± 7 × 10 3 μm 2) and 1.9-fold ( p < 0.001) as compared to WT controls (97 ± 15 × 10 3 μm 2). In vitro cholesterol efflux experiments established that combined deletion of Abcg1 and Apoe leads to a larger attenuation of macrophage cholesterol efflux to HDL as compared to single knockouts. Conclusions Single deletion of macrophage Abcg1 or Apoe does lead to a moderate non-significant increase in atherosclerotic lesion development as tested by ANOVA, while combined deletion of Abcg1 and Apoe induces a more dramatic and significant increase in atherosclerosis. Our results indicate an additive, independent effect for both macrophage Abcg1 and Apoe in the prevention of atherosclerosis.

Abstract 1721: Deletion of Both Macrophage ABCG1 and ApoE Independently Promotes Atherosclerotic Lesion Development in LDL Receptor Knockout Mice

Objective ATP-binding cassette transporter G1 (ABCG1) and apolipoprotein E (apoE) play a role in macrophage cholesterol efflux and consequently the development of atherosclerosis. Although a possible interaction between ABCG1 and apoE in cholesterol efflux was postulated, the combined action of these proteins in atherosclerosis is still unclear. Methods and Results LDL receptor knockout (KO) mice were transplanted with bone marrow from ABCG1/apoE double KO (dKO) mice, their respective single knockouts, and wild-type (WT) controls. After feeding a high-fat/high-cholesterol diet for 6 weeks, no differences were found in serum lipid levels. However, the mean atherosclerotic lesion area in dKO transplanted animals (187 ± 18 × 10 3 μ m 2 ) was 1.4-fold (p &lt; 0.01) increased compared to single knockouts (ABCG1 KO: 138 ± 5 × 10 3 μm 2 ; apoE KO: 131 ± 7 × 10 3 μm 2 ) and 1.9-fold (p&lt; 0.001) as compared to WT controls (97 ± 15 × 10 3 μm 2 ). In vitro cholesterol efflux experiments confirmed that combined deletion of ABCG1 and apoE resulted in a larger attenuation of macrophage cholesterol efflux to HDL as compared to single knockouts. Conclusions Deletion of macrophage ABCG1 or apoE does lead to a moderate increase in atherosclerotic lesion development while combined deletion of ABCG1 and apoE induces a more dramatic increase in atherosclerosis. These results indicate an added, independent effect for both macrophage ABCG1 and apoE in atherosclerosis.

Macrophage apolipoprotein-E knockdown modulates caspase-3 activation without altering sensitivity to apoptosis

Apolipoprotein-E (apoE) expression may be associated with apoptosis resistance. Since macrophages constitutively synthesize apoE we speculated that this may contribute to apoptosis resistance. Using siRNA, human monocyte derived macrophage (hMDM) apoE mRNA and protein was reduced by 97% and 61%, respectively. ApoE knockdown increased staurosporine-induced caspase-3 activation by 78% without altering cell survival or apoptosis as assessed by TUNEL analysis and morphological changes. This result was confirmed using murine bone marrow derived macrophages (mBMDM) from apoE null and wild type mice. In these experiments, staurosporine-induced caspase-3 activation was increased by 49% in apoE null compared to wild type mBMDM and this was not associated with differences in TUNEL signal, annexin-V binding or DNA fragmentation. ApoE is also important for cholesterol transport and macrophage cholesterol can regulate apoptosis. Knockdown of hMDM apoE inhibited basal cholesterol efflux by 20% without altering apolipoprotein-AI mediated cholesterol efflux over 24 h. Similarly, in apoE null mBMDM a non significant trend for a 16% reduction in basal cholesterol efflux was observed as compared to wild type mBMDM. In conclusion, apoE expression modulates capase-3 activity, but this has no significant impact on sensitivity to apoptosis and only a moderate impact on basal cholesterol efflux.

Liver X receptors inhibit human monocyte-derived macrophage foam cell formation by inhibiting fluid-phase pinocytosis of LDL

Liver X receptors (LXRs) are ligand-activated transcription factors involved in the control of lipid metabolism and inflammation. Several studies have recently shown that LXRs promote reverse cholesterol transport and inhibit atherosclerosis. Our study investigated whether LXRs affect macrophage uptake of LDL by human monocyte-derived macrophages. We have previously shown that human monocytes differentiated into macrophages with macrophage-colony-stimulating factor (M-CSF) constitutively take up large amounts of native LDL by receptor-independent, fluid-phase pinocytosis. In the research reported here, human monocytes were differentiated to macrophages in the presence of M-CSF with or without the LXR agonists T0901317 or 22(R)-hydroxycholesterol. Then, macrophages were incubated with native (125)I-LDL to determine LDL uptake. T0901317 and 22(R)-hydroxycholesterol inhibited (125)I-LDL uptake by 68 +/- 1% and 69 +/- 2%, respectively, and decreased pinocytotic vacuoles in the macrophages. (125)I-BSA uptake, a measure of fluid-phase pinocytosis, and (125)I-LDL uptake were the same, and T0901317 treatment inhibited uptake of both to the same degree. T0901317 did not affect receptor-mediated uptake of acetylated LDL, showing that the LXR effect is specific for fluid-phase pinocytosis of lipoproteins. Our results show that LXRs downregulate macrophage pinocytosis of LDL. The findings reveal an additional new mechanism by which LXR agonists may inhibit macrophage cholesterol accumulation and atherosclerosis, namely, by inhibiting macrophage uptake of LDL.

Effect of macrophage ApoE on atherosclerosis in LDL-receptor deficient mice

Apolipoprotein E (ApoE) plays an important role in the development of atherosclerosis. Previous studies provide evidence for an atheroprotective role of ApoE in mouse models on the ApoE deficient (ApoE−/−) background. However, it is not clear whether this is also true on the LDL-receptor deficient (LDLR−/−) background. Transgenic mice carrying hApoE coding sequences in a chicken lysozyme expression cassette were generated. Transgene expression was directed into macrophages, expressing low levels of hApoE. Expression of the hApoE transgene was not sufficient to correct hypercholesterolemia. However, lesion area at the brachiocephalic artery (BCA) was significantly reduced (−72%) in female hApoE transgenic mice on the LDLR−/− background. This was associated with increased cholesterol efflux in macrophages of transgenic animals on the ApoE−/− background. We conclude that over-expression of ApoE in macrophages might be useful as a therapeutic principle for the prevention of atherosclerosis.

Inflammatory Signaling Pathways Regulating ApoE Gene Expression in Macrophages

The atheroprotective role of apolipoprotein E (apoE) is well established. During inflammation, expression of apoE in macrophages is reduced leading to enhanced atheromatous plaque development. In the present study, we investigated the signaling pathways involved in the repression of apoE gene expression in response to lipopolysaccharide (LPS) treatment, a condition that mimics the inflammatory stress, in mouse macrophages RAW 264.7. We identified Tpl-2 and MEKK1 as the kinases that are primarily responsible for the down-regulation of apoE promoter activity by LPS. Using a dominant negative form of IkappaB, we established that Tpl-2 and MEKK1 signaling pathways converge to NF-kappaB acting on the apoE core promoter -55/+73. In addition to NF-kappaB activation, LPS also activated c-Jun via its phosphorylation by JNK. The activity of the apoE promoter was repressed by c-Jun, whereas small interference RNA-mediated inhibition of endogenous c-Jun expression reversed the inhibitory effect of Tpl-2 on the apoE promoter. Transfection experiments and DNA binding assays showed that the binding site for c-Jun is in the -55/+73 region of the apoE promoter. Finally, we showed that LPS inhibited apoE gene expression via activation of the Tpl-2/MEK/ERK pathway acting on a different apoE promoter region. In summary, LPS represses apoE gene expression in macrophages via signaling pathways that involve the upstream kinases Tpl-2 and MEKK1, the intermediate mitogen-activated protein kinases ERK and JNK, and the downstream transcription factors AP-1 and NF-kappaB that inhibit the apoE promoter activity via distinct regions.

A Pathway-Dependent on ApoE, ApoAI, and ABCA1 Determines Formation of Buoyant High-Density Lipoprotein by Macrophage Foam Cells

ABCA1-dependent and ABCA1-independent pathways may operate in high-density lipoprotein formation by macrophages secreting apolipoprotein (apo) E. We examined the impact of ABCA1 on apoE-mediated efflux from cholesterol-enriched macrophages. Without acceptors, wild-type, ABCA1-/-, and apoE-/- macrophages released 5.7%+/-0.3%, 1.8%+/-0.1%, and 2.3%+/-0.2% of their cholesterol, and the LXR agonist, TO-901317, enhanced efflux by 137%, 10%, and 20%. Although similar amounts of apoE were secreted from ABCA1-/- and wild-type cells, apoE from ABCA1-/- cells was only partially phospholipidated and floated at density > 1.21 g/mL, whereas apoE from wild-type cells floated at density of 1.09 to 1.17 g/mL and paralleled the density of cholesterol. With apoAI, LXR stimulation increased efflux by 139% and 86% from wild-type and apoE-/- cells, resulting in a large difference in efflux (29.5%+/-0.2% versus 17.0%+/-0.5%). The density of apoE and cholesterol from wild-type cells did not change with apoAI, and most apoAI floated at density > or = 1.17 g/mL. In apoE-/- cells, apoAI and cholesterol floated at similar density, but the peak fraction only contained 4 microg cholesterol/mg protein versus 18 in WT cells. Macrophage apoE requires ABCA1 for formation of high-density lipoprotein. ApoAI facilitates association of apoE with more buoyant high-density lipoprotein, suggesting that apoE, plasma apoAI, and ABCA1 operate together to optimize mobilization of macrophage cholesterol, a process critical to limiting plaque development.

Regulation of macrophage apoE secretion and sterol efflux by the LDL receptor

Factors that regulate apolipoprotein E (apoE) secretion by macrophages will have important effects on vessel wall lipid flux and atherosclerosis. Macrophages express the LDL receptor, which binds apoE with high affinity and could thereby affect the net secretion of apoE from macrophages. In these studies, we demonstrate that treatment of J774 macrophages transfected to constitutively express a human apoE3 cDNA with simvastatin, to increase LDL receptor activity, reduces the secretion of apoE. To further examine the relationship between LDL receptor expression and apoE secretion from macrophages, mouse peritoneal macrophages (MPMs) were isolated from mice with constitutively high expression of human LDL receptor to increase overall LDL receptor expression by 2- to 3-fold. Cells with increased LDL receptor expression also showed reduced apoE secretion compared with MPMs with basal LDL receptor expression. The effect of changes in LDL receptor expression on apoE secretion was isoform-specific, with greater reduction of apoE4 compared with apoE3 secretion and no reduction of apoE2 secretion, paralleling the known affinity of each isoform for LDL receptor binding. The effect of the LDL receptor on apoE secretion for each isoform was further reflected in LDL receptor-dependent changes in apoE-mediated cholesterol efflux. These results establish a regulatory interaction between two branches of macrophage sterol homeostatic pathways that could facilitate a rapid response to changes in macrophage sterol content relative to need.

Regulation of macrophage apoE secretion and sterol efflux by the LDL receptor

Regulation of macrophage apoE secretion and sterol efflux by the LDL receptor

Macrophage Phospholipid Transfer Protein Deficiency and ApoE Secretion

PLTP and apoE play important roles in lipoprotein metabolism and atherosclerosis. It is known that formation of macrophage-derived foam cells (which highly express PLTP and apoE) is the critical step in the process of atherosclerosis. We investigated the relationship between PLTP and apoE in macrophages and the atherogenic relevance in a mouse model. We transplanted PLTP-deficient mouse bone marrow into apoE-deficient mice (PLTP-/- --> apoE-/-), creating a mouse model with PLTP deficiency and apoE expression exclusively in the macrophages. We found that PLTP-/- --> apoE-/- mice have significantly lower PLTP activity, compared with controls (WT --> apoE-/-; 20%, P<0.01). On a Western diet, PLTP-/- --> apoE-/- mice have significantly lower plasma apoE than that of WT --> apoE-/- mice (63%, P<0.001), and PLTP-deficient macrophages secrete significantly less apoE than WT macrophages (44%, P<0.01). Moreover, PLTP-/- --> apoE-/- mice have significantly higher plasma cholesterol (98%, P<0.001) and phospholipid (107%, P<0.001) than that of WT --> apoE-/- mice, thus increasing atherosclerotic lesions in the aortic arch and root (403%, P<0.001), as well as the entire aorta (298%, P<0.001). Macrophage PLTP deficiency causes a significant reduction of apoE secretion from the cells, and this in turn promotes the accumulation of cholesterol in the circulation and accelerates the development of atherosclerosis.

Decreased Atherosclerosis in Low-Density Lipoprotein Receptor Knockout Mice Transplanted With Abcg1 −/− Bone Marrow

Recent studies indicate that the ATP-binding cassette transporter ABCG1 can promote cholesterol efflux from macrophages to high-density lipoprotein. This study was designed to assess the in vivo role of macrophage ABCG1 in atherosclerosis. Bone marrow from Abcg1-/- mice was transplanted into irradiated Ldlr-/- recipients, and atherosclerosis was evaluated by aortic root assay after 7 or 11 weeks of feeding on a Western diet. After 7 weeks, there was no difference in lesion area in mice receiving either wild-type or Abcg1-/- bone marrow, whereas after 11 weeks, lesion area was moderately but significantly reduced in Abcg1-/- recipients. ABCG1-deficient peritoneal macrophages showed induction of several liver X receptor target genes, such as Abca1 and Srebp1c, and a dramatic increase in apolipoprotein E (apoE) protein both in cell media and lysates, without parallel change in apoE mRNA. Abca1 knockdown prevented the increase in apoE secretion but had minimal effects on apoE accumulation in cell lysates of Abcg1-/- macrophages. Plasma apoE levels were markedly increased in recipients of Abcg1-/- bone marrow. These studies reveal an inverse relationship between Abcg1 expression and apoE accumulation and secretion in macrophages. The reduced atherosclerosis in recipients of Abcg1-deficient bone marrow may be explained by induction of Abca1 and an associated increase in macrophage apoE secretion.

Genetics and molecular biology: macrophage ACAT depletion – mechanisms of atherogenesis

Genetics and molecular biology: macrophage ACAT depletion – mechanisms of atherogenesis

Macrophage Apolipoprotein E Reduces Atherosclerosis and Prevents Premature Death in Apolipoprotein E and Scavenger Receptor–Class BI Double-Knockout Mice

Mice null for both apolipoprotein (apo)E and scavenger receptor (SR)-BI (DKO) develop severe hypercholesterolemia, occlusive coronary atherosclerosis, myocardial infarction, and premature death. The current study examines the ability of macrophage apoE to improve the dyslipidemia, reduce atherosclerosis, and rescue the lethal phenotype of DKO mice. Initially, bone marrow transplantation (BMT) was unsuccessful, because the DKO mice died from a rapidly fatal anemia 3 to 5 days after lethal irradiation. Therefore, probucol was used to rescue the DKO mice during BMT and was discontinued 2-weeks after BMT, allowing successful reconstitution with donor marrow. Twelve male apoE(-/-)SR-BI(-/-) mice fed 0.5% probucol in a chow diet were lethally irradiated and transplanted with either wild-type (WT) or DKO bone marrow. Two-weeks after BMT, apoE was detected in serum in WT-->DKO mice, and mean serum cholesterol levels were reduced by 70% versus DKO-->DKO mice. Lipoprotein profiles and HDL subpopulations in WT-->DKO mice were similar to apoE(+/+)SR-BI(-/-)-->DKO mice and resembled those of SR-BI(-/-) mice. In WT-->DKO mice, aortic atherosclerosis was reduced by 88% to 90% versus DKO-->DKO mice. Furthermore, the DKO-->DKO mice died &8 weeks after BMT, whereas WT-->DKO mice exhibited a life span >40 weeks after BMT. Macrophage apoE is able to rescue the lethal phenotype of apoE(-/-)SR-BI(-/-) mice by improving the dyslipidemia and dramatically reducing atherosclerotic lesion development.

Synthetic LXR agonists increase LDL in CETP species

Liver X receptor (LXR) nuclear receptors regulate the expression of genes involved in whole body cholesterol trafficking, including absorption, excretion, catabolism, and cellular efflux, and possess both anti-inflammatory and antidiabetic actions. Accordingly, LXR is considered an appealing drug target for multiple indications. Synthetic LXR agonists demonstrated inhibition of atherosclerosis progression in murine genetic models; however, these and other studies indicated that their major undesired side effect is an increase of plasma and hepatic triglycerides. A significant impediment to extrapolating results with LXR agonists from mouse to humans is the absence in mice of cholesteryl ester transfer protein, a known LXR target gene, and the upregulation in mice but not humans of cholesterol 7alpha-hydroxylase. To better predict the human response to LXR agonism, two synthetic LXR agonists were examined in hamsters and cynomolgus monkeys. In contrast to previously published results in mice, neither LXR agonist increased HDL-cholesterol in hamsters, and similar results were obtained in cynomolgus monkeys. Importantly, in both species, LXR agonists increased LDL-cholesterol, an unfavorable effect not apparent from earlier murine studies. These results reveal additional problems associated with current synthetic LXR agonists and emphasize the importance of profiling compounds in preclinical species with a more human-like LXR response and lipoprotein metabolism.

Interplay Between Apolipoprotein E and Scavenger Receptor Class B Type I Controls Coronary Atherosclerosis and Lifespan in the Mouse

The mouse has become a standard model for the study of atherosclerosis induced by extreme dyslipidemia in genetically modified strains such as the apolipoprotein E (apoE)–deficient or the LDL-receptor–deficient mouse. Although the process of plaque formation appears to repeat the fundamental steps of the human disease, it has been notoriously difficult to reproduce in the mouse the clinical consequences of atherosclerosis, such as myocardial infarction and stroke, that so commonly in patients signal plaque rupture or total lumen occlusion.1 The apoE-deficient mouse is a particularly interesting model because of its susceptibility to atherosclerotic lesions spanning from the aortic sinus to the abdominal aorta and involving the brachiocephalic and carotid arteries as well.2,3 It has recently been reported that the brachiocephalic plaque in the apoE-deficient mouse is prone to rupture on feeding a high-fat diet for periods as short as 8 weeks,4 but even in this setting of exaggerated hypercholesterolemia, the aortic sinus and coronary tree do not appear to be susceptible to plaque rupture. Scavenger receptor class B type I (SR-BI) is a scavenger-type receptor expressed by many tissues and accepted by consensus to represent the docking receptor for HDL in the liver. SR-BI-deficient mice have raised HDL levels and increased atherosclerosis susceptibility on fat feeding.5 ApoE and SR-BI are both expressed, among other cells, by the hepatocyte and the macrophage. ApoE is an efficient ligand for receptor-mediated lipoprotein removal by the liver, but it is also a strong acceptor of cellular cholesterol. As such, it may control the rate of foam cell formation in the vessel wall. Similarly, SR-BI channels cholesterol in and out of the cell, and its abundant expression by the macrophage may regulate cholesterol homeostasis in arterial foam cells. Although apoE and SR-BI play crucial roles in lipoprotein metabolism and have been …