ApoE regulates hematopoietic stem cell proliferation, monocytosis, and monocyte accumulation in atherosclerotic lesions in mice

Importance of insulin resistance to vascular repair and regeneration

Metabolic insulin resistance is apparent across a spectrum of clinical disorders, including obesity and diabetes, and is characterized by an adverse clustering of cardiovascular risk factors related to abnormal cellular responses to insulin. These disorders are becoming increasingly prevalent and represent a major global public health concern because of their association with significant increases in atherosclerosis-related mortality. Endogenous repair mechanisms are thought to retard the development of vascular disease, and a growing evidence base supports the adverse impact of the insulin-resistant phenotype upon indices of vascular repair. Beyond the impact of systemic metabolic changes, emerging data from murine studies also provide support for abnormal insulin signaling at the level of vascular cells in retarding vascular repair. Interrelated pathophysiological factors, including reduced nitric oxide bioavailability, oxidative stress, altered growth factor activity, and abnormal intracellular signaling, are likely to act in conjunction to impede vascular repair while also driving vascular damage. Understanding of these processes is shaping novel therapeutic paradigms that aim to promote vascular repair and regeneration, either by recruiting endogenous mechanisms or by the administration of cell-based therapies.

Liver ABCA1 deletion in LDLrKO mice does not impair macrophage reverse cholesterol transport or exacerbate atherogenesis

OBJECTIVE

Hepatic ATP binding cassette transporter A1 (ABCA1) expression is critical for maintaining plasma high-density lipoprotein (HDL) concentrations, but its role in macrophage reverse cholesterol transport and atherosclerosis is not fully understood. We investigated atherosclerosis development and reverse cholesterol transport in hepatocyte-specific ABCA1 knockout (HSKO) mice in the low-density lipoprotein (LDL) receptor KO (LDLrKO) C57BL/6 background.

APPROACH AND RESULTS

Male and female LDLrKO and HSKO/LDLrKO mice were switched from chow at 8 weeks of age to an atherogenic diet (10% palm oil, 0.2% cholesterol) for 16 weeks. Chow-fed HSKO/LDLrKO mice had HDL concentrations 10% to 20% of LDLrKO mice, but similar very low-density lipoprotein and LDL concentrations. Surprisingly, HSKO/LDLrKO mice fed the atherogenic diet had significantly lower (40% to 60%) very low-density lipoprotein, LDL, and HDL concentrations (50%) compared with LDLrKO mice. Aortic surface lesion area and cholesterol content were similar for both genotypes of mice, but aortic root intimal area was significantly lower (20% to 40%) in HSKO/LDLrKO mice. Although macrophage (3)H-cholesterol efflux to apoB lipoprotein-depleted plasma was 24% lower for atherogenic diet-fed HSKO/LDLrKO versus LDLrKO mice, variation in percentage efflux among individual mice was <2-fold compared with a 10-fold variation in plasma HDL concentrations, suggesting that HDL levels, per se, were not the primary determinant of plasma efflux capacity. In vivo reverse cholesterol transport, resident peritoneal macrophage sterol content, biliary lipid composition, and fecal cholesterol mass were similar between both genotypes of mice.

CONCLUSIONS

The markedly reduced plasma HDL pool in HSKO/LDLrKO mice is sufficient to maintain macrophage reverse cholesterol transport, which, along with reduced plasma very low-density lipoprotein and LDL concentrations, prevented the expected increase in atherosclerosis.

Cardioprotective functions of HDLs

Multiple human population studies have established the concentration of high density lipoprotein (HDL) cholesterol as an independent, inverse predictor of the risk of having a cardiovascular event. Furthermore, HDLs have several well-documented functions with the potential to protect against cardiovascular disease. These include an ability to promote the efflux of cholesterol from macrophages in the artery wall, inhibit the oxidative modification of low density lipoproteins (LDLs), inhibit vascular inflammation, inhibit thrombosis, promote endothelial repair, promote angiogenesis, enhance endothelial function, improve diabetic control, and inhibit hematopoietic stem cell proliferation. There are undoubtedly other beneficial functions of HDLs yet to be identified. The HDL fraction in human plasma is heterogeneous, consisting of several subpopulations of particles of varying size, density, and composition. The functions of the different HDL subpopulations remain largely unknown. Given that therapies that increase the concentration of HDL cholesterol have varying effects on the levels of specific HDL subpopulations, it is of great importance to understand how distribution of different HDL subpopulations contribute to the potentially cardioprotective functions of this lipoprotein fraction. This review summarizes current understanding of the relationship of HDL subpopulations to their cardioprotective properties and highlights the gaps in current knowledge regarding this important aspect of HDL biology.

Inhibition of monocyte chemotaxis by VB-201, a small molecule lecinoxoid, hinders atherosclerosis development in ApoE⁻/⁻ mice

OBJECTIVE

Monocytes are motile cells which sense inflammatory stimuli and subsequently migrate to sites of inflammation. Key players in host defense, monocytes have nevertheless been implicated as requisite mediators of several chronic inflammatory diseases. Inhibition of monocyte chemotaxis is therefore an attractive anti-inflammatory strategy. Oxidized phospholipids (OxPL) are native regulators of inflammation, yet their direct effect on monocyte chemotaxis is poorly defined. In this study, we investigated the direct effect of natural and synthetic phospholipids on monocyte chemotaxis.

METHODS

Exploring various phospholipids using in vitro chemotaxis assays, we found that the natural phospholipid 1-palmitoyl-2-glutaryl phosphatidylcholine (PGPC) can decrease monocyte chemotaxis by 50%, while other tested OxPL had no effect. We generated a library of synthetic OxPL designated lecinoxoids, which was screened for anti-inflammatory properties.

RESULTS AND CONCLUSIONS

VB-201, a small-molecule lecinoxoid, exhibited up to 90% inhibition of monocyte chemotaxis in vitro. Molecular analysis revealed that the effect of VB-201 was not restricted to a specific chemotactic ligand or receptor, and resulted from inhibition of signaling pathways required for monocyte chemotaxis. Interestingly, VB-201 did not inhibit monocyte adhesion or phagocytosis and had no effect on chemotaxis of CD4(+) T-cells or neutrophils. In vivo, oral treatment with VB-201 reduced monocyte migration in a peritonitis model and inhibited atheroma development in ApoE(-/-) mice, without affecting cholesterol or triglyceride levels. Our findings highlight a novel role played by native and synthetic phospholipids in regulation of monocyte chemotaxis. The data strengthen the involvement of phospholipids as key signaling molecules in inflammatory settings and demonstrate their potential therapeutic applicability.

Inducible ApoE gene repair in hypomorphic ApoE mice deficient in the low-density lipoprotein receptor promotes atheroma stabilization with a human-like lipoprotein profile

OBJECTIVE

To study atherosclerosis regression in mice after plasma lipid reduction to moderately elevated apolipoprotein B (apoB)-lipoprotein levels.

APPROACH AND RESULTS

Chow-fed hypomorphic Apoe mice deficient in low-density lipoprotein receptor expression (Apoe(h/h)Ldlr(-/-)Mx1-cre mice) develop hyperlipidemia and atherosclerosis. These mice were studied before and after inducible cre-mediated Apoe gene repair. By 1 week, induced mice displayed a 2-fold reduction in plasma cholesterol and triglyceride levels and a decrease in the non-high-density lipoprotein:high-density lipoprotein-cholesterol ratio from 87%:13% to 60%:40%. This halted atherosclerotic lesion growth and promoted macrophage loss and accumulation of thick collagen fibers for up to 8 weeks. Concomitantly, blood Ly-6C(high) monocytes were decreased by 2-fold but lesional macrophage apoptosis was unchanged. The expression of several genes involved in extracellular matrix remodeling and cell migration was changed in lesional macrophages 1 week after Apoe gene repair. However, mRNA levels of numerous genes involved in cholesterol efflux and inflammation were not significantly changed at this time point.

CONCLUSIONS

Restoring apoE expression in Apoe(h/h)Ldlr(-/-)Mx1-cre mice resulted in lesion stabilization in the context of a human-like ratio of non-high-density lipoprotein:high-density lipoprotein-cholesterol. Our data suggest that macrophage loss derived in part from reduced blood Ly-6C(high) monocytes levels and genetic reprogramming of lesional macrophages.

Pathophysiological role of neutrophils in acute myocardial infarction

The pathogenesis of acute myocardial infarction is known to be mediated by systemic, intraplaque and myocardial inflammatory processes. Among different immune cell subsets, compelling evidence now indicates a pivotal role for neutrophils in acute coronary syndromes. Neutrophils infiltrate coronary plaques and the infarcted myocardium and mediate tissue damage by releasing matrix-degrading enzymes and reactive oxygen species. In addition, neutrophils are also involved in post-infarction adverse cardiac remodelling and neointima formation after angioplasty. The promising results obtained in preclinical modelswith pharmacological approaches interfering with neutrophil recruitment or function have confirmed the pathophysiological relevance of these immune cells in acute coronary syndromes and prompted further studies of these therapeutic interventions. This narrative review will provide an update on the role of neutrophils in acute myocardial infarction and on the pharmacological means that were devised to prevent neutrophil-mediated tissue damage and to reduce post-ischaemic outcomes.

Hyperglycemia promotes myelopoiesis and impairs the resolution of atherosclerosis

Diabetes is a major risk factor for atherosclerosis. Although atherosclerosis is initiated by deposition of cholesterol-rich lipoproteins in the artery wall, the entry of inflammatory leukocytes into lesions fuels disease progression and impairs resolution. We show that diabetic mice have increased numbers of circulating neutrophils and Ly6-C(hi) monocytes, reflecting hyperglycemia-induced proliferation and expansion of bone marrow myeloid progenitors and release of monocytes into the circulation. Increased neutrophil production of S100A8/S100A9, and its subsequent interaction with the receptor for advanced glycation end products on common myeloid progenitor cells, leads to enhanced myelopoiesis. Treatment of hyperglycemia reduces monocytosis, entry of monocytes into atherosclerotic lesions, and promotes regression. In patients with type 1 diabetes, plasma S100A8/S100A9 levels correlate with leukocyte counts and coronary artery disease. Thus, hyperglycemia drives myelopoiesis and promotes atherogenesis in diabetes.

Pegylation of high-density lipoprotein decreases plasma clearance and enhances antiatherogenic activity

RATIONALE

Infusions of apolipoprotein AI (apoAI), mimetic peptides, or high-density lipoprotein (HDL) remain a promising approach for the treatment of atherosclerotic coronary disease. However, rapid clearance leads to a requirement for repeated administration of large amounts of material and limits effective plasma concentrations.

OBJECTIVE

Because pegylation of purified proteins is commonly used as a method to increase their half-life in the circulation, we determined whether pegylation of apoAI or HDL would increase its plasma half-life and in turn its antiatherogenic potential.

METHODS AND RESULTS

Initial pegylation attempts using lipid-poor apoAI showed a marked tendency to form multi-pegylated (PEG) species with reduced ability to promote cholesterol efflux from macrophage foam cells. However, pegylation of human holo-HDL or reconstituted phospholipid/apoAI particles (rHDL) led to selective N-terminal monopegylation of apoAI with full preservation of cholesterol efflux activity. The plasma clearance of PEG-rHDL was estimated after injection into hypercholesterolemic Apoe-/- mice; the half-life of pegylated PEG-apoAI after injection of PEG-rHDL was increased ≈7-fold compared with apoAI in nonpegylated rHDL. In comparison with nonpegylated rHDL, infusion of PEG-rHDL (40 mg/kg) into hypercholesterolemic Apoe-/- mice led to more pronounced suppression of bone marrow myeloid progenitor cell proliferation and monocytosis, as well as reduced atherosclerosis and a stable plaque phenotype.

CONCLUSIONS

We describe a novel method for effective monopegylation of apoAI in HDL particles, in which lipid binding seems to protect against pegylation of key functional residues. Pegylation of apoAI in rHDL markedly increases its plasma half-life and enhances antiatherogenic properties in vivo.

Hypercholesterolemia and reduced HDL-C promote hematopoietic stem cell proliferation and monocytosis: studies in mice and FH children

Previous studies have shown that mice with defects in cellular cholesterol efflux show hematopoietic stem cell (HSPC) and myeloid proliferation, contributing to atherogenesis. We hypothesized that the combination of hypercholesterolemia and defective cholesterol efflux would promote HSPC expansion and leukocytosis more prominently than either alone. We crossed Ldlr(-/-) with Apoa1(-/-) mice and found that compared to Ldlr(-/-) mice, Ldlr(-/-)/Apoa1(+/-) mice, with similar LDL-cholesterol levels but reduced HDL-cholesterol (HDL-C) levels, had expansion of HSPCs, monocytosis and neutrophilia. Ex vivo studies showed that HSPCs expressed high levels of Ldlr, Scarb1 (Srb1), and Lrp1 and were able to take up both native and oxidized LDL. Native LDL directly stimulated HSPC proliferation, while co-incubation with reconstituted HDL attenuated this effect. We also assessed the impact of HDL-C levels on monocytes in children with familial hypercholesterolemia (FH) (n = 49) and found that subjects with the lowest level of HDL-C, had increased monocyte counts compared to the mid and higher HDL-C levels. Overall, HDL-C was inversely correlated with the monocyte count. These data suggest that in mice, a balance of cholesterol uptake and efflux mechanisms may be one factor in driving HSPC proliferation and monocytosis. Higher monocyte counts in children with FH and low HDL-cholesterol suggest a similar pattern in humans.

Paradoxical protection from atherosclerosis and thrombosis in a mouse model of sickle cell disease

Sickle cell disease (SCD) is associated with vascular complications including premature stroke. The role of atherothrombosis in these vascular complications is unclear. To determine the effect of SCD on atherosclerosis and thrombosis, mice with SCD along with controls were generated by transplantation of bone marrow from mice carrying the homozygous sickle cell mutation (Hbb(hβs/hβs) ) or wild-type mice (Hbb(+/+) ) into C57BL6/J or apolipoprotein E deficient (Apoe(-/-) ) recipient mice. At the time of sacrifice, 23-28 weeks following bone marrow transplantation, anaemia, reticulocytosis, and splenomegaly were present in mice receiving Hbb(hβs/hβs) bone marrow compared with control mice. Analysis of atherosclerosis involving the aortic root revealed reduced atherosclerotic lesion area with reduced macrophage content and increased collagen content in Apoe(-/-) , Hbb(hβs/hβs) mice compared to Apoe(-/-) , Hbb(+/+) mice. In a carotid thrombosis model, the time to thrombosis was prolonged in Hbb(hβs/hβs) mice compared to Hbb(+/+) mice. This apparent protective effect of SCD on atherosclerosis and thrombosis was diminished by inhibition of heme oxygenase-1 (HMOX1) using zinc protoporphyrin IX. We conclude that SCD in mice is paradoxically protective against atherosclerosis and thrombosis, highlighting the complexity of vascular events in SCD. This protective effect is at least partially mediated by induction of HMOX1.

Cholesterol efflux in megakaryocyte progenitors suppresses platelet production and thrombocytosis

Platelets play a key role in atherogenesis and its complications. Both hypercholesterolemia and increased platelet production promote athero-thrombosis; however, a potential link between altered cholesterol homeostasis and platelet production has not been explored. Transplantation of bone marrow (BM) deficient in ABCG4, a transporter of unknown function, into Ldlr^−/− mice resulted in thrombocytosis, accelerated thrombosis and atherosclerosis. While not detected in lesions, Abcg4 was highly expressed in BM megakaryocyte progenitors (MkP). Abcg4^−/− MkPs displayed defective cholesterol efflux to HDL, increased cell surface levels of thrombopoietin (TPO) receptor (c-MPL) and enhanced proliferation. This appeared to reflect disruption of the negative feedback regulation of c-MPL levels and signaling by E3 ligase c-CBL and cholesterol-sensing LYN kinase. HDL infusions reduced platelet counts in Ldlr^−/− mice and in a mouse model of myeloproliferative neoplasm, in a completely ABCG4-dependent fashion. HDL infusions may offer a novel approach to reducing athero-thrombotic events associated with increased platelet production.

Deficiency of ATP-binding cassette transporters A1 and G1 in macrophages increases inflammation and accelerates atherosclerosis in mice

RATIONALE

Plasma high-density lipoprotein levels are inversely correlated with atherosclerosis. Although it is widely assumed that this is attributable to the ability of high-density lipoprotein to promote cholesterol efflux from macrophage foam cells, direct experimental support for this hypothesis is lacking.

OBJECTIVE

To assess the role of macrophage cholesterol efflux pathways in atherogenesis.

METHODS AND RESULTS

We developed mice with efficient deletion of the ATP-binding cassette transporters A1 and G1 (ABCA1 and ABCG1) in macrophages (MAC-ABC(DKO) mice) but not in hematopoietic stem or progenitor populations. MAC-ABC(DKO) bone marrow (BM) was transplanted into Ldlr(-/-) recipients. On the chow diet, these mice had similar plasma cholesterol and blood monocyte levels but increased atherosclerosis compared with controls. On the Western-type diet, MAC-ABC(DKO) BM-transplanted Ldlr(-/-) mice had disproportionate atherosclerosis, considering they also had lower very low-density lipoprotein/low-density lipoprotein cholesterol levels than controls. ABCA1/G1-deficient macrophages in lesions showed increased inflammatory gene expression. Unexpectedly, Western-type diet-fed MAC-ABC(DKO) BM-transplanted Ldlr(-/-) mice displayed monocytosis and neutrophilia in the absence of hematopoietic stem and multipotential progenitor cells proliferation. Mechanistic studies revealed increased expressions of machrophage colony stimulating factor and granulocyte colony stimulating factor in splenic macrophage foam cells, driving BM monocyte and neutrophil production.

CONCLUSIONS

These studies show that macrophage deficiency of ABCA1/G1 is proatherogenic likely by promoting plaque inflammation and uncover a novel positive feedback loop in which cholesterol-laden splenic macrophages signal BM progenitors to produce monocytes, with suppression by macrophage cholesterol efflux pathways.

The complexity of arterial classical monocyte recruitment

Accumulation of classical monocytes is imperative for the progression of atherosclerosis. Hence, therapeutic interference with mechanisms of lesional monocyte recruitment, the primary mechanism controlling macrophage accumulation, may allow for targeting atheroprogression and its clinical complications. Here, we review the important role of classical monocytes in atheroprogression as well as their routes of arterial recruitment. We specifically highlight the role of cell adhesion molecules as well as of platelet-derived chemokines and neutrophil-borne alarmins.

Making a difference: monocyte heterogeneity in cardiovascular disease

Monocytes are frequently described as bone marrow-derived precursors of macrophages. Although many studies support this view, we now appreciate that monocytes neither develop exclusively in the bone marrow nor give rise to all macrophages and dendritic cells. In addition to differentiating to specific leukocyte populations, monocytes, as monocytes, are functionally and ontogenically heterogeneous. In this review we will focus on the development and activity of monocytes and their subsets in mice (Ly-6 C(high/low)) and humans (CD14(+/dim/-) CD16(+/-)) in the context of atherosclerosis and its complications.

Hypercholesterolemia links hematopoiesis with atherosclerosis

Atherosclerosis is characterized by the progressive accumulation of lipids and leukocytes in the arterial wall. Leukocytes such as macrophages accumulate oxidized lipoproteins in the growing atheromata and give rise to foam cells, which can then contribute to the necrotic core of lesions. Lipids and leukocytes also interact in other important ways. In experimental models, systemic hypercholesterolemia is associated with severe neutrophilia and monocytosis. Recent evidence indicates that cholesterol-sensing pathways control the proliferation of hematopoietic stem-cell progenitors. Here we review some of the studies that are forging this particular link between metabolism and inflammation, and propose several strategies that could target this axis for the treatment of cardiovascular disease.

Regulation of hematopoietic stem and progenitor cell mobilization by cholesterol efflux pathways

Intact cholesterol homeostasis helps to maintain hematopoietic stem and multipotential progenitor cell (HSPC) quiescence. Mice with defects in cholesterol efflux pathways due to deficiencies of the ATP binding cassette transporters ABCA1 and ABCG1 displayed a dramatic increase in HSPC mobilization and extramedullary hematopoiesis. Increased extramedullary hematopoiesis was associated with elevated serum levels of G-CSF due to generation of IL-23 by splenic macrophages and dendritic cells. This favored hematopoietic lineage decisions toward granulocytes rather than macrophages in the bone marrow leading to impaired support for osteoblasts and decreased Cxcl12/SDF-1 production by mesenchymal progenitors. Greater HSPC mobilization and extramedullary hematopoiesis were reversed by raising HDL levels in Abca1(-/-)Abcg1(-/-) and Apoe(-/-) mice or in a mouse model of myeloproliferative neoplasm mediated by Flt3-ITD mutation. Our data identify a role of cholesterol efflux pathways in the control of HSPC mobilization. This may translate into therapeutic strategies for atherosclerosis and hematologic malignancies.

Greasing the way: the ABCs of HSPC efflux from the marrow

In this issue of Cell Stem Cell, Westerterp et al. demonstrate that cholesterol flux through splenic phagocytes regulates hematopoietic stem and multipotential progenitor cell mobilization from the bone marrow. These exciting findings suggest new avenues for therapy of myeloproliferative disorders and atherosclerosis.

Distinct functions of chemokine receptor axes in the atherogenic mobilization and recruitment of classical monocytes

We used a novel approach of cytostatically induced leucocyte depletion and subsequent reconstitution with leucocytes deprived of classical (inflammatory/Gr1^hi) or non-classical (resident/Gr1^lo) monocytes to dissect their differential role in atheroprogression under high-fat diet (HFD). Apolipoprotein E-deficient (Apoe^−/−) mice lacking classical but not non-classical monocytes displayed reduced lesion size and macrophage and apoptotic cell content. Conversely, HFD induced a selective expansion of classical monocytes in blood and bone marrow. Increased CXCL1 levels accompanied by higher expression of its receptor CXCR2 on classical monocytes and inhibition of monocytosis by CXCL1-neutralization indicated a preferential role for the CXCL1/CXCR2 axis in mobilizing classical monocytes during hypercholesterolemia. Studies correlating circulating and lesional classical monocytes in gene-deficient Apoe^−/− mice, adoptive transfer of gene-deficient cells and pharmacological modulation during intravital microscopy of the carotid artery revealed a crucial function of CCR1 and CCR5 but not CCR2 or CX₃CR1 in classical monocyte recruitment to atherosclerotic vessels. Collectively, these data establish the impact of classical monocytes on atheroprogression, identify a sequential role of CXCL1 in their mobilization and CCR1/CCR5 in their recruitment.

Expanded granulocyte/monocyte compartment in myeloid-specific triple FoxO knockout increases oxidative stress and accelerates atherosclerosis in mice

RATIONALE

Increased neutrophil and monocyte counts are often associated with an increased risk of atherosclerosis, but their relationship to insulin sensitivity is unknown.

OBJECTIVE

To investigate the contribution of forkhead transcription factors (FoxO) in myeloid cells to neutrophil and monocyte counts, atherosclerosis, and systemic insulin sensitivity.

METHODS AND RESULTS

Genetic ablation of the 3 genes encoding FoxO isoforms 1, 3a, and 4, in myeloid cells resulted in an expansion of the granulocyte/monocyte progenitor compartment and was associated with increased atherosclerotic lesion formation in low-density lipoprotein receptor knockout mice. In vivo and ex vivo studies indicate that FoxO ablation in myeloid cells increased generation of reactive oxygen species. Accordingly, treatment with the antioxidant N-acetyl-l-cysteine reversed the phenotype, normalizing atherosclerosis.

CONCLUSIONS

Our data indicate that myeloid cell proliferation and oxidative stress can be modulated via the FoxO branch of insulin receptor signaling, highlighting a heretofore-unknown link between insulin sensitivity and leukocytosis that can affect the predisposition to atherosclerosis.

Liver X receptor and peroxisome proliferator-activated receptor as integrators of lipid homeostasis and immunity

Lipid metabolism has emerged as an important modulator of innate and adaptive immune cell fate and function. The lipid-activated transcription factors peroxisome proliferator-activated receptor (PPAR) α, β/δ, γ and liver X receptor (LXR) are members of the nuclear receptor superfamily that have a well-defined role in regulating lipid homeostasis and metabolic diseases. Accumulated evidence over the last decade indicates that PPAR and LXR signaling also influence multiple facets of inflammation and immunity, thereby providing important crosstalk between metabolism and immune system. Herein, we provide a brief introduction to LXR and PPAR biology and review recent discoveries highlighting the importance of PPAR and LXR signaling in the modulation of normal and pathologic states of immunity. We also examine advances in our mechanistic understanding of how nuclear receptors impact immune system function and homeostasis. Finally, we discuss whether LXRs and PPARs could be pharmacologically manipulated to provide novel therapeutic approaches for modulation of the immune system under pathologic inflammation or in the context of allergic and autoimmune disease.

Leukocyte behavior in atherosclerosis, myocardial infarction, and heart failure

Cardiovascular diseases claim more lives worldwide than any other. Etiologically, the dominant trajectory involves atherosclerosis, a chronic inflammatory process of lipid-rich lesion growth in the vascular wall that can cause life-threatening myocardial infarction (MI). Those who survive MI can develop congestive heart failure, a chronic condition of inadequate pump activity that is frequently fatal. Leukocytes (white blood cells) are important participants at the various stages of cardiovascular disease progression and complication. This Review will discuss leukocyte function in atherosclerosis, MI, and heart failure.

Cholesterol efflux: a novel regulator of myelopoiesis and atherogenesis

Monocytosis and neutrophilia are well-established risk factors for atherosclerosis and seem to play a causative role in lesion development. Studies in mice with defects in cholesterol efflux pathways have identified novel roles for the ATP-binding cassette transporter A1, ATP-binding cassette transporter G1, and apolipoprotein E in suppressing hematopoietic stem cell proliferation, mobilization, and the production of monocytes and neutrophils in the bone marrow. In addition, stem cell mobilization to the spleen initiates extramedullary hematopoiesis, which acts as a monocytic reservoir. Increased monocyte and neutrophil levels drive atherogenesis and its complications. Increasing high-density lipoprotein levels and cholesterol efflux can reverse excessive myelopoiesis and stem cell mobilization, suggesting a novel antiatherogenic effect of some forms of high-density lipoprotein elevation. After a myocardial infarction, splenic Ly-6C(hi) monocyte populations are sustained by a second wave of stem cell mobilization from the bone marrow and continue to enter atheroma, accelerating atherogenesis. Because activation of cholesterol efflux pathways can inhibit stem cell proliferation, mobilization, and monocyte production, this may provide a rationale for boosting high-density lipoprotein levels after a myocardial infarction to prevent reocclusion.

Imaging macrophage development and fate in atherosclerosis and myocardial infarction

Macrophages are central regulators of disease progression in both atherosclerosis and myocardial infarction (MI). In atherosclerosis, macrophages are the dominant leukocyte population that influences lesional development. In MI, which is caused by atherosclerosis, macrophages accumulate readily and have important roles in inflammation and healing. Molecular imaging has grown considerably as a field and can reveal biological process at the molecular, cellular and tissue levels. Here, we explore how various imaging modalities, from intravital microscopy in mice to organ-level imaging in patients, are contributing to our understanding of macrophages and their progenitors in cardiovascular disease.

Liver X receptor modulates diabetic retinopathy outcome in a mouse model of streptozotocin-induced diabetes

Endothelial progenitor cells (EPCs), critical for mediating vascular repair, are dysfunctional in a hyperglycemic and/or hypercholesterolemic environment. Their dysfunction contributes to the progression of diabetic macro- and microvascular complications. Activation of "cholesterol-sensing" nuclear receptors, the liver X receptors (LXRα/LXRβ), protects against atherosclerosis by transcriptional regulation of genes important in promoting cholesterol efflux and inhibiting inflammation. We hypothesized that LXR activation with a synthetic ligand would correct diabetes-induced EPC dysfunction and improve diabetic retinopathy. Studies were performed in streptozotocin (STZ)-injected DBA/2J mice fed a high-fat Western diet (DBA/STZ/WD) and treated with the LXR agonist GW3965 and in LXRα(-/-), LXRβ(-/-), and LXRα/β(-/-) mice. Retinas were evaluated for number of acellular capillaries and glial fibrillary acidic protein (GFAP) immunoreactivity. Bone marrow EPCs were analyzed for migratory function and gene expression. Compared with vehicle-treated DBA/STZ/WD mice, GW3965 treated mice showed fewer acellular capillaries and reduced GFAP expression. These mice also exhibited enhanced EPC migration and restoration of inflammatory and oxidative stress genes toward nondiabetic levels. LXRα(-/-), LXRβ(-/-), and LXRα/β(-/-) mice developed acellular capillaries and EPC dysfunction similar to the DBA/STZ/WD mice. These studies support a key role for LXR in retinal and bone marrow progenitor dysfunction associated with type 1 diabetes. LXR agonists may represent promising pharmacologic targets for correcting retinopathy and EPC dysfunction.

A diseased bone marrow fuels atherosclerosis in diabetes

Diabetes accelerates atherosclerosis through shortage of vascular regenerative cells derived from the bone marrow (BM). In addition, diabetes shifts the differentiation of BM progenitor cells to pro-calcific and smooth muscle phenotypes. In a paper published in Atherosclerosis, Fledderus et al. demonstrate that the accelerated atherosclerosis in diabetic ApoE(-/-) mice is associated with an increased amount of BM-derived smooth muscle cells in the plaques. The role of ApoE in the regulation of vascular BM progenitors may explain inconsistencies in the literature on the contribution of extraparietal cells to atherosclerotic lesions. Herein, the pathophysiological meaning of a deranged kinetic of smooth muscle progenitor cells in diabetes is discussed.

Heterodimerization with the prostacyclin receptor triggers thromboxane receptor relocation to lipid rafts

OBJECTIVE

Prostacyclin and thromboxane mediate opposing cardiovascular actions through receptors termed IP and TP, respectively. When dimerized with IP, the TP shifts to IP-like function. IP localizes to cholesterol-enriched membrane rafts, but TP and IPTP heterodimer localization is not defined. We examined these receptors' membrane localization and the role of rafts in receptor function.

METHODS AND RESULTS

Microdomain distribution of IP, TP, and IPTP heterodimers was examined in COS-7 cells by measuring energy transfer from renilla luciferase-fused receptors to fluorescently labeled rafts. IP raft association was confirmed. TP was raft excluded, but redistributed to rafts upon dimerization with IP. Signaling of the IP and IPTP heterodimer, but not TP alone, was suppressed after raft disruption by cholesterol depletion. Cholesterol enrichment also selectively suppressed IP and IPTP function. Native IP and IPTP signaling in smooth muscle cells and macrophages were similarly sensitive to cholesterol manipulation, whereas macrophages from hypercholesterolemic mice displayed suppressed IP and IPTP function.

CONCLUSIONS

IP and TP function within distinct microdomains. Raft incorporation of TP in the IPTP heterodimer likely facilitates its signaling shift. We speculate that changes in IP and IPTP signaling after perturbation of membrane cholesterol may contribute to cardiovascular disease associated with hypercholesterolemia.

Cholesterol accumulation inhibits ER to Golgi transport and protein secretion: studies of apolipoprotein E and VSVGt

Cholesterol excess is typical of various diseases including atherosclerosis. We have investigated whether cholesterol accumulation in the ER (endoplasmic reticulum) can inhibit exit of vesicular cargo and secretion of proteins by studying apoE (apolipoprotein E), a significant glycoprotein in human health and disease. CHO (Chinese hamster ovary) cells expressing human apoE under a cholesterol-independent promoter incubated with cholesterol-cyclodextrin complexes showed increased levels of cellular free and esterified cholesterol, inhibition of SREBP-2 (sterol-regulatory-element-binding protein 2) processing, and a mild induction of ER stress, indicating significant accumulation of cholesterol in the ER. Secretion of apoE was markedly inhibited by cholesterol accumulation, and similar effects were observed in cells enriched with lipoprotein-derived cholesterol and in primary human macrophages. Removal of excess cholesterol by a cyclodextrin vehicle restored apoE secretion, indicating that the transport defect was reversible. That cholesterol impaired protein trafficking was supported by the cellular accumulation of less sialylated apoE glycoforms, and by direct visualization of altered ER to Golgi transport of thermo-reversible VSVG (vesicular stomatitis virus glycoprotein) linked to GFP (green fluorescent protein). We conclude that intracellular accumulation of cholesterol in the ER reversibly inhibits protein transport and secretion. Strategies to correct ER cholesterol may restore homoeostatic processes and intracellular protein transport in conditions characterized by cholesterol excess.

Hematopoietic stem/progenitor cell proliferation and differentiation is differentially regulated by high-density and low-density lipoproteins in mice

Rationale

Hematopoietic stem/progenitor cells (HSPC) are responsible for maintaining the blood system as a result of their self-renewal and multilineage differentiation capacity. Recently, studies have suggested that HDL cholesterol may inhibit and impaired cholesterol efflux may increase HSPC proliferation and differentiation.

Objectives

We hypothesized that LDL may enhance HSPC proliferation and differentiation while HDL might have the opposing effect which might influence the size of the pool of inflammatory cells.

Methods and Results

HSPC number and function were studied in hypercholesterolemic LDL receptor knockout (LDLr^−/−) mice on high fat diet. Hypercholesterolemia was associated with increased frequency of HSPC, monocytes and granulocytes in the peripheral blood (PB). In addition, an increased proportion of BM HSPC was in G₂M of the cell cycle, and the percentage of HSPC and granulocyte-macrophage progenitors (GMP) increased in BM of LDLr^−/− mice. When BM Lin-Sca-1+cKit+ (i.e. “LSK”) cells were cultured in the presence of LDL in vitro we also found enhanced differentiation towards monocytes and granulocytes. Furthermore, LDL promoted lineage negative (Lin−) cells motility. The modulation by LDL on HSPC differentiation into granulocytes and motility was inhibited by inhibiting ERK phosphorylation. By contrast, when mice were infused with human apoA-I (the major apolipoprotein of HDL) or reconstituted HDL (rHDL), the frequency and proliferation of HSPC was reduced in BM in vivo. HDL also reversed the LDL-induced monocyte and granulocyte differentiation in vitro.

Conclusion

Our data suggest that LDL and HDL have opposing effects on HSPC proliferation and differentiation. It will be of interest to determine if breakdown of HSPC homeostasis by hypercholesterolemia contributes to inflammation and atherosclerosis progression.

Anti-inflammatory functions of apolipoprotein A-I and high-density lipoprotein are preserved in trimeric apolipoprotein A-I

Raising high-density lipoprotein (HDL) levels is proposed as an attractive target to treat cardiovascular disease. However, a number of clinical studies examining the effect of HDL-raising therapies have been prematurely halted due to futility. Therefore there is a need for alternative therapies. Infusion of reconstituted HDL (rHDL) particles is still considered as a viable approach to increasing HDL levels. In this study we have profiled the anti-inflammatory effects of a trimeric-HDL particle. We show that trimeric apoA-I and rHDL particles promote cholesterol efflux to a similar rate as native apoA-I particles in both ABCA1-dependent and -independent pathways. Trimeric particles inhibited ICAM-1 and VCAM-1 expression and the ability of the endothelium to capture monocytes under shear flow. Monocyte activation, CD11b-dependent adhesion, and monocyte recruitment under shear flow conditions were perturbed by the trimeric particles. Our data suggest that trimeric rHDL particles can be constructed without any loss of function, preserving the anti-inflammatory effects of HDL that are key to its in vivo actions.

Apolipoprotein E regulation of myeloid cell plasticity in atherosclerosis

PURPOSE OF REVIEW

Apolipoprotein (apo) E is a multifunctional protein that has long been recognized for its ability to safeguard against atherosclerosis. Among its pleiotropic roles known to suppress atherosclerosis, mechanisms by which apoE regulates cells of the immune system have remained elusive. Because atherosclerosis is a chronic inflammatory disease that remains on the rise, understanding in more detail how apoE controls immune cell activation and function is of much interest.

RECENT FINDINGS

Literature reported in the past year introduces apoE as a regulator of monocyte and macrophage plasticity. Through signals delivered by its interaction with cell surface receptors, apoE has been shown to influence the polarity and inflammatory phenotypes of the macrophage. By promoting cellular cholesterol efflux in a cell autonomous manner and through its ability to enhance HDL function in hyperlipidemic plasma, apoE is now known to suppress atherosclerosis by controlling myeloid cell proliferation, monocyte activation and their capacity to infiltrate the vascular wall. Lastly, the structural basis for apoE isoform-specific effects in macrophage dysfunction and atherosclerosis susceptibility is beginning to emerge.

SUMMARY

Collectively, these findings introduce a new dimension to our understanding of how apoE links lipoprotein biology to monocyte and macrophage function in atherosclerosis susceptibility.

Apo E4 and lipoprotein-associated phospholipase A2 synergistically increase cardiovascular risk

OBJECTIVE

Apolipoprotein E (apoE) has been implicated as conveying increased risk for coronary artery disease (CAD). Previous studies suggest a role of apoE as a modulator of immune response and inflammatory properties. We hypothesized that the presence of apo E4 is associated with an increased inflammatory burden in subjects with CAD as compared to subjects without CAD.

METHODS

ApoE genotypes, systemic (C-reactive protein [CRP], fibrinogen, serum amyloid-A [SAA]) and vascular inflammatory markers (Lipoprotein-associated phospholipase A(2) [Lp-PLA(2)] and pentraxin-3 [PTX-3]) were assessed in 324 Caucasians and 208 African Americans, undergoing coronary angiography.

RESULTS

For both ethnic groups, Lp-PLA(2) index, an integrated measure of Lp-PLA(2) mass and activity, increased significantly and stepwise across apoE isoforms (P = 0.009 and P = 0.026 for African Americans and Caucasians respectively). No differences were found for other inflammatory markers tested (CRP, fibrinogen, SAA and PTX-3). For the top cardiovascular score tertile, apo E4 carriers had a significantly higher level of Lp-PLA(2) index in both ethnic groups (P = 0.027 and P = 0.010, respectively).

CONCLUSION

The presence of the apo E4 isoform was associated with a higher level of Lp-PLA(2) index, a marker of vascular inflammation. Our results suggest that genetic variation at the apoE locus may impact cardiovascular disease risk through enhanced vascular inflammation.

Inflammatory Ly-6C(hi) monocytes play an important role in the development of severe transplant arteriosclerosis in hyperlipidemic recipients

Objective

Transplant arteriosclerosis (TA) restricts long-term survival of heart transplant recipients. Although the role of monocyte/macrophages is well established in native atherosclerosis, it has been studied to a much lesser extent in TA. Plasma cholesterol is the most important non-immunologic risk factor for development of TA but the underlying mechanisms are largely unknown. We hypothesized that monocyte/macrophages might play an important role in the pathogenesis of TA under hyperlipidemic conditions.

Methods

We studied TA in fully mismatched arterial allografts transplanted into hyperlipidemic ApoE^−/− recipients compared to wild-type controls. The recruitment of distinct monocyte populations into the grafts was tracked by in vivo labelling with fluorescent microspheres. We used antibody-mediated depletion protocols to dissect the relative contribution of T lymphocytes and monocytes to disease development.

Results

In the hyperlipidemic environment the progression of TA was highly exacerbated and the inflammatory CD11b⁺CD115⁺Ly-6C^hi monocytes were preferentially recruited into the neointima. The number of macrophage-derived foam cells present in the grafts strongly correlated with plasma cholesterol and disease severity. Depletion of Ly-6C^hi monocytes and neutrophils significantly inhibited macrophage accumulation and disease progression. The accelerated monocyte recruitment occurs through a T cell-independent mechanism, as T cell depletion did not influence macrophage accumulation into the grafts.

Conclusions

Our study identifies for the first time the involvement of inflammatory Ly-6C^hi monocytes into the pathogenesis of TA, particularly in conditions of hyperlipidemia. Targeted therapies modulating the recruitment and activation of these cells could potentially delay coronary allograft vasculopathy and improve long-term survival of heart transplant recipients.

Multiple roles for neutrophils in atherosclerosis

Because of their rare detection in atherosclerotic lesions, the involvement of neutrophils in the pathophysiology of atherosclerosis has been largely denied. However, over the past couple of years, studies have provided convincing evidence for the presence of neutrophils in atherosclerotic plaques and further revealed the causal contribution of neutrophils during various stages of atherosclerosis. This review describes mechanisms underlying hyperlipidemia-mediated neutrophilia and how neutrophils may enter atherosclerotic lesions. It also highlights possible mechanisms of neutrophil-driven atherogenesis and plaque destabilization. Knowledge of the contribution of neutrophils to atherosclerosis will allow for exploration of new avenues in the treatment of atherogenesis and atherothrombosis.

New roles of HDL in inflammation and hematopoiesis

High-density lipoprotein (HDL) levels are inversely associated with coronary heart disease due to HDL's ability to transport excess cholesterol in arterial macrophages to the liver for excretion [i.e., reverse cholesterol transport (RCT)]. However, recent advances highlight additional atheroprotective roles for HDL beyond bulk cholesterol removal from cells through RCT. By promoting cellular free cholesterol (FC) efflux, HDL and its apolipoproteins (apoA-I and apoE) decrease plasma membrane FC and lipid raft content in immune and hematopoietic stem cells, decreasing inflammatory and cell proliferation signaling pathways. HDL and apoA-I also dampen inflammatory signaling pathways independent of cellular FC efflux. In addition, HDL lipid and protein cargo provide protection against parasitic and bacterial infection, endothelial damage, and oxidant toxicity. Here, current knowledge is reviewed regarding the role of HDL and its apolipoproteins in regulating cellular cholesterol homeostasis, highlighting recent advances on novel functions and mechanisms by which HDLs regulate inflammation and hematopoiesis.

Liver LXRα expression is crucial for whole body cholesterol homeostasis and reverse cholesterol transport in mice

Liver X receptors (LXRα and LXRβ) are important regulators of cholesterol and lipid metabolism, and their activation has been shown to inhibit cardiovascular disease and reduce atherosclerosis in animal models. Small molecule agonists of LXR activity are therefore of great therapeutic interest. However, the finding that such agonists also promote hepatic lipogenesis has led to the idea that hepatic LXR activity is undesirable from a therapeutic perspective. To investigate whether this might be true, we performed gene targeting to selectively delete LXRα in hepatocytes. Liver-specific deletion of LXRα in mice substantially decreased reverse cholesterol transport, cholesterol catabolism, and cholesterol excretion, revealing the essential importance of hepatic LXRα for whole body cholesterol homeostasis. Additionally, in a pro-atherogenic background, liver-specific deletion of LXRα increased atherosclerosis, uncovering an important function for hepatic LXR activity in limiting cardiovascular disease. Nevertheless, synthetic LXR agonists still elicited anti-atherogenic activity in the absence of hepatic LXRα, indicating that the ability of agonists to reduce cardiovascular disease did not require an increase in cholesterol excretion. Furthermore, when non-atherogenic mice were treated with synthetic LXR agonists, liver-specific deletion of LXRα eliminated the detrimental effect of increased plasma triglycerides, while the beneficial effect of increased plasma HDL was unaltered. In sum, these observations suggest that therapeutic strategies that bypass the liver or limit the activation of hepatic LXRs should still be beneficial for the treatment of cardiovascular disease.

Bone marrow transplantation as an established approach for understanding the role of macrophages in atherosclerosis and the metabolic syndrome

PURPOSE OF REVIEW

Bone marrow transplantation (BMT) technology is a firmly established tool for studying atherosclerosis. Only recently it is helping us to understand the inflammatory mechanisms leading to the development of obesity, insulin resistance and type 2 diabetes. Here we review the use of BMT as a tool for studying the metabolic syndrome.

RECENT FINDINGS

Bone marrow-derived cells, and particularly monocytes and macrophages, have been a major subject in the study of atherogenesis, and they are highly amenable for research purposes because of their application in bone marrow transplantations. For example, the many pathways studied using BMT have helped unmask ABC transporters as the genes controlling reverse cholesterol transport and foam cell formation, as well as other genes like CCR2 and IκBα controlling leukocyte development, migration and activation. The invasion of leukocytes, not only in the vessel wall, but also in adipose tissue and liver, shares many common mechanisms relevant to atherosclerosis and metabolic diseases.

SUMMARY

BMT is an efficient and versatile tool for assessing the roles of specific genes that are restricted to hematopoietic cells, and especially the monocytes and macrophages in metabolic syndrome and its related pathologies.

Regulation of macrophage and dendritic cell responses by their lineage precursors

Tissue macrophages (Mø) and dendritic cells (DC) are thought to derive from hematopoietic stem cells, which exist in the bone marrow and generate intermediate precursor populations with increasingly restricted lineage potentials. There exists several precursors committed to the Mø and DC lineages; these cells exhibit distinct tropism and function and respond differentially in pathophysiologic conditions. In this review, we consider experimental contexts in which Mø and DC responses in tissue are not only dictated by the local environment, but also by the quantity and quality of newly recruited lineage precursor cells. Consequently, we discuss whether therapeutic control of Mø and DC responses in tissue may be achieved through manipulation of their lineage precursors.

Beyond genome-wide association studies: the usefulness of mouse genetics in understanding the complex etiology of atherosclerosis

The development of population-based genome-wide association studies has led to the rapid identification of large numbers of genetic variants associated with coronary artery disease (CAD) and related traits. Together with large-scale gene-centric studies, at least 35 loci associated with CAD per se have been identified with replication. The majority of these associations are with common single-nucleotide polymorphisms exhibiting modest effects on relative risk. The modest nature of the effects, coupled with ethical/practical constraints associated with human sampling, makes it difficult to answer important questions beyond gene/locus localization and allele frequency via human genetic studies. Questions related to gene function, disease-causing mechanism(s), and effective interventions will likely require studies in model organisms. The use of the mouse model for further detailed studies of CAD-associated loci identified by genome-wide association studies is highlighted herein.

ApoE controls the interface linking lipids and inflammation in atherosclerosis

Atherosclerosis is a chronic inflammatory disease of the arterial walls that often leads to myocardial infarction and/or stroke. Hypercholesterolemia and an imbalance of peripheral leukocyte counts, leading to arterial leukocyte infiltration, are considered independent risk factors for atherosclerosis. However, in this issue of the JCI, Murphy and colleagues identify a mechanistic link between hypercholesterolemia, leukocytosis, and the subsequent development of atherosclerotic lesions in mice. These findings could pave the way for the development of novel treatment strategies to control leukocyte homeostasis and atherosclerosis.