Consequences and Therapeutic Implications of Macrophage Apoptosis in Atherosclerosis

LipoCardium: Endothelium-directed cyclopentenone prostaglandin-based liposome formulation that completely reverses atherosclerotic lesions

Atherosclerosis is a multifactorial inflammatory disease of blood vessels which decimates one in every three people in industrialized world. Despite the important newest clinical approaches, currently available strategies (e.g. nutritional, pharmacological and surgical) may only restrain the worsening of vascular disease. Since antiproliferative cyclopentenone prostaglandins (CP-PGs) are powerful anti-inflammatory agents, we developed a negatively charged liposome-based pharmaceutical formulation (LipoCardium) that specifically direct CP-PGs towards the injured arterial wall cells of atherosclerotic mice. In the blood stream, LipoCardium delivers its CP-PG contents only into activated arterial wall lining cells due to the presence of antibodies raised against vascular cell adhesion molecule-1 (VCAM-1), which is strongly expressed upon inflammation by endothelial cells and macrophage-foam cells as well. After 4 months in a high-lipid diet, all low-density lipoprotein receptor-deficient adult control mice died from myocardium infarction or stroke in less than 2 weeks, whereas LipoCardium-treated (2 weeks) animals (still under high-lipid diet) completely recovered from vascular injuries. In vitro studies using macrophage-foam cells suggested a tetravalent pattern for LipoCardium action: anti-inflammatory, antiproliferative (and pro-apoptotic only to foam cells), antilipogenic and cytoprotector (via heat-shock protein induction). These astonishing cellular effects were accompanied by a marked reduction in arterial wall thickness, neointimal hyperplasia and lipid accumulation, while guaranteed lifespan to be extended to the elderly age. Our findings suggest that LipoCardium may be safely tested in humans in a near future and may have conceptual implications in atherosclerosis therapy.

NR4A nuclear receptors in atherosclerosis and vein-graft disease

Nur77, Nurr1, and NOR-1 form the NR4A subfamily of the nuclear hormone receptor superfamily of transcription factors and have been described in the regulation of differentiation, proliferation, apoptosis, and survival of many different cell types. The expression of NR4A nuclear receptors in vascular pathologies has only recently been revealed, after which studies on the functional involvement of NR4A receptors in vascular disease were initiated. This review summarizes our current view on involvement of Nur77, Nurr1, and NOR-1 in atherosclerotic vascular disease and discusses NR4A function in vascular response to injury.

Enhanced Immune System Activation and Arterial Inflammation Accelerates Atherosclerosis in Lupus-Prone Mice

OBJECTIVE

Premature atherosclerosis is a characteristic feature of systemic lupus erythematosus, a prototypic autoimmune disease. The principle cellular and molecular mechanisms which underlie such accelerated atherosclerosis are indeterminate.

METHODS AND RESULTS

The pathophysiology of lupus-mediated atherogenesis was evaluated in a novel animal model involving transplantation of bone marrow cells from the lupus prone strain gld into Ldl-r(-/-) mice. Diet-induced atherogenesis in lethally-irradiated Ldl-r(-/-) mice transplanted with gld bone marrow cells resulted in accelerated atherosclerosis (+65%) as compared with control mice transplanted with wild-type marrow cells. Enhanced atherogenesis was associated with enhanced activation of both B and T lymphocytes and with arterial inflammation involving endothelial cell activation, monocyte recruitment, and accumulation of apoptotic debris, macrophages, and CD4 T cells, but was independent of plasma lipid levels and renal function.

CONCLUSIONS

Our data support the contention that despite the absence of both disturbed cholesterol homeostasis and renal dysfunction in autoimmune gld-->Ldl-r(-/-) mice, lupus disease induces enhanced activation of the immune system and acts locally on the vasculature to induce inflammation, together with accumulation of apoptotic debris, macrophages, and CD4 T cells, thereby accelerating plaque progression.

Cytotoxic phospholipid oxidation products. Cell death from mitochondrial damage and the intrinsic caspase cascade

Phospholipid oxidation products accumulate in the necrotic core of atherosclerotic lesions, in apoptotic cells, and circulate in oxidized low density lipoprotein. Phospholipid oxidation generates toxic products, but little is known about which specific products are cytotoxic, their receptors, or the mechanism(s) that induces cell death. We find the most common phospholipid oxidation product of oxidized low density lipoprotein, phosphatidylcholine with esterified sn-2-azelaic acid, induced apoptosis at low micromolar concentrations. The synthetic ether phospholipid hexadecyl azelaoyl phosphatidylcholine (HAzPC) was rapidly internalized, and overexpression of PLA2g7 (PAF acetylhydrolase) that specifically hydrolyzes such oxidized phospholipids suppressed apoptosis. Internalized HAzPC associated with mitochondria, and cytochrome c, and apoptosis-inducing factor escaped from mitochondria to the cytoplasm and nucleus, respectively, in cells exposed to HAzPC. Isolated mitochondria exposed to HAzPC rapidly swelled and released cytochrome c and apoptosis-inducing factor. Other phospholipid oxidation products induced swelling, but HAzPC was the most effective and was twice as effective as its diacyl homolog. Cytoplasmic cytochrome c completes the apoptosome, and activated caspase 9 and 3 were present in cells exposed to HAzPC. Irreversible inhibition of caspase 9 blocked downstream caspase 3 activation and prevented apoptosis. Mitochondrial damage initiated this apoptotic cascade, because overexpression of Bcl-X(L), an anti-apoptotic protein localized to mitochondria, blocked cytochrome c escape and apoptosis. Thus, exogenous phospholipid oxidation products target intracellular mitochondria to activate the intrinsic apoptotic cascade.

The macrophage at the crossroads of insulin resistance and atherosclerosis

The macrophage has emerged as an important player in the pathogenesis of both atherosclerosis and insulin resistance. Cross-talk between inflammatory macrophages and adipocytes may be involved in insulin resistance in peripheral tissues. Defective insulin signaling in cells of the arterial wall including macrophages may promote the development of atherosclerosis. Insulin resistant macrophages are more susceptible to endoplasmic reticulum stress and apoptosis in response to various stimuli such as nutrient deprivation, free cholesterol loading, and oxidized LDL. Increased apoptosis of insulin resistant macrophages and impaired phagocytic clearance of apoptotic cells by insulin resistant macrophages in atherosclerotic lesions may lead to enhanced postapoptotic necrosis, larger lipid-rich cores, increased inflammation, and more complex vulnerable plaques.

Pivotal advance: macrophages become resistant to cholesterol-induced death after phagocytosis of apoptotic cells

One of the most important functions of macrophages is the phagocytosis of apoptotic cells (ACs). ACs deliver large amounts membrane-derived cholesterol to phagocytes, which, if not handled properly, can be cytotoxic. In atherosclerosis, where the ACs are cholesterol-loaded, this situation is exaggerated, because the ACs deliver both endogenous membrane cholesterol and stored lipoprotein-derived cholesterol. To examine how phagocytes handle this very large amount of cholesterol, we incubated macrophage phagocytes with cholesterol-loaded ACs. Our results show that the phagocytes call into play a number of cellular responses to protect them from cholesterol-induced cytotoxicity. First, through efficient trafficking of the internalized AC-derived cholesterol to acyl-CoA:cholesterol acyltransferase (ACAT) in the endoplasmic reticulum, phagocytes efficiently esterify the cholesterol and thus prevent its toxic effects. However, the phagocytes show no signs of cytotoxicity even when ACAT is rendered dysfunctional, as might occur in advanced atherosclerotic lesions. Under these conditions, the phagocytes remain viable through massive efflux of AC-derived cholesterol. Remarkably, these phagocytes still show a survival response even when high cholesterol levels are maintained in the post-phagocytosis period by subsequent incubation with atherogenic lipoproteins, as also may occur in atheromata. In this case, death in phagocytes is prevented by activation of survival pathways involving PI-3 kinase/Akt and NF-kappaB. Thus, macrophages that have ingested ACs successfully employ three survival mechanisms -- cholesterol esterification, massive cholesterol efflux, and cell-survival signaling. These findings have implications for macrophage physiology in both AC clearance and atherosclerotic plaque progression.

Elimination of Neoangiogenesis for Plaque Stabilization

Emerging data suggest that intraplaque hemorrhage is critical in promoting atherosclerotic lesion instability. Because red blood cell membranes are a rich source of free cholesterol and accumulated red blood cells within plaques promote inflammation, intraplaque hemorrhage is associated with expansion of the necrotic core. Plaque hemorrhage results from the development of immature neointimal vasa vasorum. Therefore, it is proposed that molecular therapies designed to eliminate pathologic neovascularization within developing lesions will interrupt the process of hemorrhage and decrease the rate of necrotic core expansion. The elimination of intraplaque neovascularization would involve targeting of pre-existing and new vessel development. The concept of vascular regression has met some success in other neovascular-dependent diseases, including macular degeneration and malignancies. The efficacy of this novel approach is dependent on gaining critical knowledge of the environment required to support development and maturation of the vasa vasorum within varying plaque types. A multitargeted approach involving selective local antiangiogenic agents should contribute to prevention of plaque progression and its clinical consequences.

Paraoxonase-2 Reduces Oxidative Stress in Vascular Cells and Decreases Endoplasmic Reticulum Stress–Induced Caspase Activation

Background— In the vascular system, elevated levels of reactive oxygen species (ROS) produce oxidative stress and predispose to the development of atherosclerosis. Therefore, it is important to understand the systems producing and those scavenging vascular ROS. Here, we analyzed the ROS-reducing capability of paraoxonase-2 (PON2) in different vascular cells and its involvement in the endoplasmic reticulum stress pathway known as the unfolded protein response.

Methods and Results— Quantitative real-time polymerase chain reaction and Western blotting revealed that PON2 is equally expressed in vascular cells and appears in 2 distinct glycosylated isoforms. By determining intracellular ROS, we show that overexpression of PON2 markedly reduced ROS, whereas its knockdown increased ROS levels significantly. Using microscopic and biochemical methods, we found PON2 mainly in the nuclear membrane and endoplasmic reticulum. Furthermore, PON2 expression was induced at both the promoter and protein levels by endoplasmic reticulum stress pathway unfolded protein response. This pathway may promote both apoptotic and survival mechanisms. Functionally, PON2 reduced unfolded protein response–accompanying oxidative stress and unfolded protein response–derived caspase activation.

Conclusion— We suggest that PON2 represents an endogenous defense mechanism against vascular oxidative stress and unfolded protein response–induced cell death, thereby contributing to the prevention of atherosclerosis.

Poly(ADP-ribose) polymerase inhibition reduces atherosclerotic plaque size and promotes factors of plaque stability in apolipoprotein E-deficient mice: effects on macrophage recruitment, nuclear factor-kappaB nuclear translocation, and foam cell death

BACKGROUND

Poly(ADP-ribose) polymerase (PARP) was suggested to play a role in endothelial dysfunction that is associated with a number of cardiovascular diseases. We hypothesized that PARP may play an important role in atherogenesis and that its inhibition may attenuate atherosclerotic plaque development in an experimental model of atherosclerosis.

METHODS AND RESULTS

Using a mouse (apolipoprotein E [ApoE](-/-)) model of high-fat diet-induced atherosclerosis, we demonstrate an association between cell death and oxidative stress-associated DNA damage and PARP activation within atherosclerotic plaques. PARP inhibition by thieno[2,3-c]isoquinolin-5-one reduced plaque number and size and altered structural composition of plaques in these animals without affecting sera lipid contents. These results were corroborated genetically with the use of ApoE(-/-) mice that are heterozygous for PARP-1. PARP inhibition promoted an increase in collagen content, potentially through an increase in tissue inhibitor of metalloproteinase-2, and transmigration of smooth muscle cells to intima of atherosclerotic plaques as well as a decrease in monocyte chemotactic protein-1 production, all of which are markers of plaque stability. In PARP-1(-/-) macrophages, monocyte chemotactic protein-1 expression was severely inhibited because of a defective nuclear factor-kappaB nuclear translocation in response to lipopolysaccharide. Furthermore, PARP-1 gene deletion not only conferred protection to foam cells against H2O2-induced death but also switched the mode of death from necrosis to apoptosis.

CONCLUSIONS

Our results suggest that PARP inhibition interferes with plaque development and may promote plaque stability, possibly through a reduction in inflammatory factors and cellular changes related to plaque dynamics. PARP inhibition may prove beneficial for the treatment of atherosclerosis.

Lactadherin deficiency leads to apoptotic cell accumulation and accelerated atherosclerosis in mice

BACKGROUND

Atherosclerosis is an immunoinflammatory disease; however, the key factors responsible for the maintenance of immune regulation in a proinflammatory milieu are poorly understood.

METHODS AND RESULTS

Here, we show that milk fat globule-EGF factor 8 (Mfge8, also known as lactadherin) is expressed in normal and atherosclerotic human arteries and is involved in phagocytic clearance of apoptotic cells by peritoneal macrophages. Disruption of bone marrow-derived Mfge8 in a murine model of atherosclerosis leads to substantial accumulation of apoptotic debris both systemically and within the developing lipid lesions. The accumulation of apoptotic material is associated with a reduction in interleukin-10 in the spleen but an increase in interferon-gamma production in both the spleen and the atherosclerotic arteries. In addition, we report a dendritic cell-dependent alteration of natural regulatory T-cell function in the absence of Mfge8. These events are associated with a marked acceleration of atherosclerosis.

CONCLUSIONS

Lack of Mfge8 in bone marrow-derived cells enhances the accumulation of apoptotic cell corpses in atherosclerosis and alters the protective immune response, which leads to an acceleration of plaque development.

Low levels of Nogo-B in human carotid atherosclerotic plaques are associated with an atheromatous phenotype, restenosis, and stenosis severity

OBJECTIVE

Reticulon-4/Nogo (Nogo-B) protects mouse arteries from lumen loss by reducing smooth muscle cell (SMC) migration and intimal thickening. Our goal was to determine plaque and circulating levels of Nogo-B in atherosclerotic and control subjects. Therefore, we studied the relationships between local Nogo-B, plaque characteristics, and clinical data in patients undergoing carotid endarterectomy.

METHODS AND RESULTS

Western blot analysis showed that endarterectomy specimens from the femoral (n=19) and carotid arteries (n=145) contained significantly less Nogo-B than nonatherosclerotic mammary arteries (n=8; P<0.003) and aortas (n=15; P=0.03). Immunohistochemistry revealed that in atherosclerotic lesions, Nogo-B was expressed by macrophage/foam cells, SMC rich, and neo-vascularized areas. Atheromatous plaques (>40% fat content) showed a significant reduction in Nogo-B expression (P=0.002). Nogo-B expression levels were significantly lower in patients with more than 90% of carotid stenosis (P=0.04) or restenotic lesions after prior carotid intervention (duplex; P=0.01). In contrast, plasmatic levels of Nogo-B (soluble Nogo-B) did not differ between atherosclerotic subjects (n=68) and risk-factor matched controls (n=63; P=0.5).

CONCLUSION

Our findings suggest that local reduction of Nogo-B in atherosclerotic tissue might contribute to plaque formation and/or instability triggering luminal narrowing. In contrast, plasma Nogo-B levels are not associated with clinically manifested atherosclerotic disease.

Induction of dendritic cell-like phenotype in macrophages during foam cell formation

Foam cell formation from monocyte-derived macrophages is a hallmark of atherosclerotic lesions. Aspects of this process can be recapitulated in vitro by exposing M-CSF-induced or platelet factor 4 (CXCL4)-induced macrophages to oxidized (ox) or minimally modified (mm) low density lipoprotein (LDL). We measured gene expression in peripheral blood mononuclear cells, monocytes, and macrophages treated with CXCL1 (GRO-α) or CCL2 (MCP-1), as well as foam cells induced by native LDL, mmLDL, or oxLDL using 22 Affymetrix gene chips. Using an advanced Bayesian error-pooling approach and a heterogeneous error model with a false discovery rate <0.05, we found 5,303 of 22,215 probe sets to be significantly regulated in at least one of the conditions. Among a subset of 917 candidate genes that were preselected for their known biological functions in macrophage foam-cell differentiation, we found that 290 genes met the above statistical criteria for significant differential expression patterns. While many expected genes were found to be upregulated by LDL and oxLDL, very few were induced by mmLDL. We also found induction of unexpected genes, most strikingly MHC-II and other dendritic cell markers such as CD11c. The gene expression patterns in response to oxLDL were similar in M-CSF-induced and CXCL4-induced macrophages. Our findings suggest that LDL and oxLDL, but not mmLDL, induce a dendritic cell-like phenotype in macrophages, suggesting that these cells may be able to present antigens and support an immune response.

Phospholipid Transfer Protein Augments Apoptosis in THP-1–Derived Macrophages Induced by Lipolyzed Hypertriglyceridemic Plasma

OBJECTIVE

Lipolysis of triglyceride-rich lipoproteins (TGRLPs) generates phospholipid-rich surface remnants and induces cytotoxic effects in adjacent vascular cells. We hypothesized that by integrating surface remnants into HDL, phospholipid transfer protein (PLTP) alleviates cytotoxicity.

METHODS AND RESULTS

To test this hypothesis and gain insight into cytotoxicity during the postprandial phase in vivo, we injected normo-TG and hyper-TG human volunteers after a standardized fat meal (postprandial sample) with heparin, thereby stimulating lipolysis (postprandial heparinized sample). Incubation of (primary) human macrophages and primary human endothelial cells with postprandial heparinized hyper-TG plasma induced pronounced cytotoxic effects that were dose dependent on the TG content of the sample. No such effects were seen with normo-TG and postprandial hyper-TG plasma. In vitro lipolysis of VLDL and chylomicrons indicated that both lipoprotein fractions can cause cytotoxicity. Interestingly, in experiments with THP-1-derived macrophages stably transfected with PLTP, PLTP substantially augmented both net phospholipid uptake and apoptotic cell death due to postprandial heparinized hyper-TG plasma. We observed that activation of caspase-3/7, poly-ADP-ribose polymerase, and enhanced bioactivity of acid sphingomyelinase may all contribute to this augmented apoptosis.

CONCLUSIONS

Our data show that lipolysis of TGRLPs and their remodelling by PLTP interact to disturb cellular phospholipid flux and intracellular signaling processes, ultimately leading to apoptosis in human macrophages and endothelial cells.

TLR4-mediated survival of macrophages is MyD88 dependent and requires TNF-alpha autocrine signalling

Modulation of macrophage survival is a critical factor in the resolution of inflammatory responses. Exposure to LPS protects innate immune cells against apoptosis, although the precise pathways responsible for prolongation of macrophage survival remain to be fully established. The goal of this study was to characterize the mechanism of TLR4-mediated survival of murine bone marrow-derived macrophages upon M-CSF withdrawal in more detail. Using a combination of knockout mice and pharmacological inhibitors allowed us to show that TLR4 and TLR2 stimulation promotes long-term survival of macrophages in a MyD88-, PI3K-, ERK-, and NF-kappaB-dependent manner. LPS-induced long-term, but not short-term, survival requires autocrine signaling via TNF-alpha and is facilitated by a general cytoprotective program, similar to that mediated by M-CSF. TLR4-mediated macrophage survival is accompanied by a remarkable up-regulation of specific cell surface markers, suggesting that LPS stimulation leads to the differentiation of macrophages toward a mixed macrophage/dendritic cell-like phenotype.

Monocyte/macrophage suppression in CD11b diphtheria toxin receptor transgenic mice differentially affects atherogenesis and established plaques

Although monocytes/macrophages are considered important in atherogenesis, their role in established plaques is unclear. For example, macrophage content is associated with plaque instability, but their loss through cell death is observed at sites of plaque rupture. To examine the role of monocytes/macrophages in atherosclerosis, we developed CD11b-diphtheria toxin (DT) receptor (DTR) transgenic mice, whereby administration of DT selectively kills monocytes/macrophages. DT treatment reduced peripheral blood monocytes and tissue macrophages and inhibited macrophage function in CD11b-DTR mice and apolipoprotein E-null (apoE(-/-)) mice transplanted with CD11b-DTR bone marrow. In atherogenesis experiments, DT markedly reduced plaque development and altered plaque composition, reducing collagen content and necrotic core formation. In mice with established plaques, acute DT treatment induced macrophage apoptosis and reduced macrophage content but did not induce plaque inflammation, thrombosis, or rupture. Furthermore, despite a 50% reduction in monocytes, chronic DT treatment of these mice did not alter plaque extent or composition, most likely because of ongoing recruitment/proliferation of monocytes with recovery of macrophage content. We conclude that monocytes/macrophages are critical to atherogenesis, but established plaques are more resistant to reductions in monocytes.

Complement C1q reduces early atherosclerosis in low-density lipoprotein receptor-deficient mice

We explored the role of the classic complement pathway in atherogenesis by intercrossing C1q-deficient mice (C1qa-/-) with low-density lipoprotein receptor knockout mice (Ldlr-/-). Mice were fed a normal rodent diet until 22 weeks of age. Aortic root lesions were threefold larger in C1qa-/-/Ldlr-/- mice compared with Ldlr-/- mice (3.72 +/- 1.0% aortic root versus 1.1 +/- 0.4%; mean +/- SEM, P < 0.001). Furthermore, the cellular composition of lesions in C1qa-/-/Ldlr-/- was more complex, with an increase in vascular smooth muscle cells. The greater aortic root lesion size in C1qa-/-/Ldlr-/- mice occurred despite a significant reduction in C5b-9 deposition per lesion unit area, suggesting the critical importance of proximal pathway activity. Apoptotic cells were readily detectable by cleaved caspase-3 staining, terminal deoxynucleotidyl transferase dUTP nick-end labeling assay, and electron microscopy in C1qa-/-/Ldlr-/-, whereas apoptotic cells were not detected in Ldlr-/- mice. This is the first direct demonstration of a role for the classic complement pathway in atherogenesis. The greater lesion size in C1qa-/-/Ldlr-/- mice is consistent with the emerging homeostatic role for C1q in the disposal of dying cells. This study suggests the importance of effective apoptotic cell removal for containing the size and complexity of early lesions in atherosclerosis.

Imaging apoptosis for detecting plaque instability: rendering death a brighter facade

The relatively poor correlation between the risk of atherosclerotic plaque rupture and the degree of luminal obstruction before this event implies a strong imperative for in vivo detection of the processes underlying progressive plaque destabilization. In addition to the morphologic characteristics, apoptosis and inflammation comprise two important indicators of plaque instability. Apoptotic macrophage death results in enlargement of the plaque necrotic core and positive vascular remodelling, whereas apoptosis of the smooth muscle cells leads to attenuation of the fibrous cap. Imaging of apoptotic cells with annexin A5 provides an opportunity for the non-invasive assessment of cell death, and hence plaque vulnerability. The clinical detection of apoptosis could therefore promote the development of novel intervention strategies.

Selective Clearance of Macrophages in Atherosclerotic Plaques by Autophagy

OBJECTIVES

The purpose of this study was to investigate whether stent-based delivery of an inhibitor of mammalian target of rapamycin (mTOR) can selectively clear macrophages in rabbit atherosclerotic plaques.

BACKGROUND

Current pharmacologic approaches to stabilize atherosclerotic plaques have only partially reduced the incidence of acute coronary syndromes and sudden death. Macrophages play a pivotal role in plaque destabilization, whereas smooth muscle cells (SMC) promote plaque stability.

METHODS

Stents eluting the mTOR inhibitor everolimus were implanted in atherosclerotic arteries of cholesterol-fed rabbits. In addition, in vitro experiments using explanted atherosclerotic segments and cultured macrophages as well as SMC were performed.

RESULTS

Stents eluting everolimus led to a marked reduction in macrophage content without altering the amount of SMC compared with polymer control stents. In vitro studies showed that everolimus treatment induced inhibition of translation in both cultured macrophages and SMC. However, cell death occurred only in macrophages and was characterized by bulk degradation of long-lived proteins, processing of microtubule-associated protein light chain 3, and cytoplasmic vacuolization, which are all markers of autophagy. Everolimus-induced autophagy was mediated by mTOR inhibition, because cell viability was not affected using tacrolimus, an mTOR-independent everolimus analog. Moreover, mTOR gene silencing was associated with selective induction of macrophage cell death. Autophagic macrophage cell death was confirmed by transmission electron microscopy both in cultured cells and in atherosclerotic explants.

CONCLUSIONS

Stent-based delivery of everolimus selectively cleared macrophages in rabbit atherosclerotic plaques by autophagy, an mTOR inhibition-dependent and novel mechanism to induce cell death in mammalian cells.

The osteoprotegerin/RANK/RANKL system: a bone key to vascular disease

Owing to the common coincidence of osteoporosis and vascular disease, pathophysiological links between both disorders have long been sought. The osteoprotegerin (OPG)/receptor activator of NF-kappaB (RANK)/receptor activator of NF-kappaB ligand (RANKL) cytokine network, a key regulatory system in bone homeostasis, has been implicated recently in vascular calcification, changes in matrix composition and diabetic macroangiopathy, aortic aneurysm development, heart failure and, most importantly, advanced atherosclerosis, plaque destabilization and manifestation of cardiovascular diseases. The concept of an active role of RANKL and OPG in vascular pathophysiology is intriguing and is gaining increasing support from both epidemiological and basic research. OPG serum level is considered to be a stable and reliable indicator of the overall activity of the OPG/RANK/RANKL axis and may find application as a biomarker of vascular risk and prognosis. RANKL in turn may be a suitable target for novel therapies. Pharmacological strategies for specific interference with the OPG/RANK/RANKL axis are currently being developed and evaluated in osteoporosis therapy.

Enhanced VDUP-1 gene expression by PPARγ agonist induces apoptosis in human macrophage

The fate and phenotype of lesion macrophages is regulated by cellular oxidative stress. Thioredoxin-1 (Trx-1) plays a major role in the regulation of cellular redox balance, with resultant effects on gene expression and cellular responses including cell growth and death. Trx-1 activity is inhibited by interaction with vitamin D-upregulated protein-1 (VDUP-1). Peroxisome proliferator-activated receptor gamma (PPARgamma) is expressed by human monocyte-derived macrophages (HMDM) and PPARgamma agonism has been reported to decrease expression of inflammatory genes and to promote apoptosis of these cells. To determine whether VDUP-1 may be involved in regulating the effects of PPARgamma agonists in macrophages, we investigated the effect of a synthetic PPARgamma agonist (GW929) on the expression of VDUP-1 in HMDM. GW929 concentration-dependently increased HMDM expression of VDUP-1 (mRNA and protein). Transfection of different fragments of the VDUP-1 promoter as well as gel shift analysis revealed the presence of functional PPARgamma response elements (PPRE) in the promoter. Under conditions in which PPAR agonism altered levels of VDUP-1, caspase-3 activity, and macrophage apoptosis were also elevated. The results suggest that PPARgamma activation stimulates apoptosis in human macrophages by altering the cellular redox balance via regulation of VDUP-1.

Critical role for classical PKC in activating Akt by phospholipase A2-modified LDL in monocytic cells

OBJECTIVE

Modification of low density lipoprotein (LDL) by phospholipases confers pro-atherogenic properties, although signalling pathways of phospholipase-modified LDL (PLA-LDL) remain obscure. We questioned whether members of the protein kinase C (PKC) family are involved in PLA-LDL-induced Akt phosphorylation and survival of THP-1 monocytic cells.

METHODS

Akt phosphorylation in THP-1 cells was monitored by Western analysis. To modulate PKC expression cells were transfected with dominant-negative enhanced green fluorescent protein linked PKCalpha (PKCalpha-EGFP K368R) and PKCbeta (PKCbeta-EGFP K371M) constructs or with siRNA specific for PKCalpha/PKCbeta using nucleofection technology. Cell survival was assessed by Annexin V/propidium iodide staining or mitochondrial membrane potential measurement with 3,3'-dihexyloxacarbocyanine iodide (DiOC(6)) using flow cytometry.

RESULTS

Inhibitors of phospholipase C (PLC) or classical PKCs as well as PKC depletion following phorbol ester treatments, blocked Akt phosphorylation in response to PLA-LDL. In contrast, phosphatidylinositol 3-kinase (PI3K) activation by PLA-LDL was insensitive to PKC inhibition. Using RNA interference to knockdown PKCalpha and overexpression of dominant-negative PKCalpha as well as PKCbeta drastically lowered Akt phosphorylation after PLA-LDL. Moreover, inhibition of PKC attenuated a PLA-LDL-induced survival response towards oxidative stress in THP-1 cells.

CONCLUSION

We show that PKCalpha and PKCbeta are critical for PLA-LDL-induced Akt phosphorylation and survival in THP-1 monocytic cells.

Combinatorial pattern recognition receptor signaling alters the balance of life and death in macrophages

Macrophage pattern recognition receptors (PRRs) play key roles in innate immunity, but they also may contribute to disease processes under certain pathological conditions. We recently showed that engagement of the type A scavenger receptor (SRA), a PRR, triggers JNK-dependent apoptosis in endoplasmic reticulum (ER)-stressed macrophages. In advanced atherosclerotic lesions, the SRA, activated JNK, and ER stress are observed in macrophages, and macrophage death in advanced atheromata leads to plaque necrosis. Herein, we show that SRA ligands trigger apoptosis in ER-stressed macrophages by cooperating with another PRR, Toll-like receptor 4 (TLR4), to redirect TLR4 signaling from prosurvival to proapoptotic. Common SRA ligands activate both TLR4 signaling and engage the SRA. The TLR4 effect results in activation of the proapoptotic MyD88-JNK branch of TLR4, whereas the SRA effect silences the prosurvival IRF-3-IFN-beta branch of TLR4. The normal cell-survival effect of LPS-induced TLR4 activation is converted into an apoptosis response by immunoneutralization of IFN-beta, and the apoptosis effect of SRA ligands is converted into a cell-survival response by reconstitution with IFN-beta. Thus, combinatorial signaling between two distinct PRRs results in a functional outcome-macrophage apoptosis that does not occur with either PRR alone. PRR-induced macrophage death may play important roles in advanced atherosclerosis and in other innate immunity-related processes in which the balance between macrophage survival and death is critical.

Selective Clearance of Macrophages in Atherosclerotic Plaques by the Protein Synthesis Inhibitor Cycloheximide

Macrophages are an essential component of unstable atherosclerotic plaques and play a pivotal role in the destabilization process. We have demonstrated previously that local delivery of the mammalian target of rapamycin (mTOR) inhibitor everolimus selectively clears macrophages in rabbit plaques. Because mTOR controls mRNA translation, inhibition of protein synthesis might induce selective macrophage cell death. We therefore investigated in the present study the effect of the protein synthesis inhibitor cycloheximide on macrophage and smooth muscle cell (SMC) viability. In vitro studies with cultured macrophages and SMCs showed that cycloheximide induced selective apoptosis of macrophages in a concentration- and time-dependent manner. Moreover, macrophages could be selectively depleted in rabbit carotid artery rings with collar-induced atherosclerotic plaques after in vitro treatment with cycloheximide. Local in vivo administration of cycloheximide via osmotic minipumps to rabbit carotid arteries with collar-induced atherosclerotic plaques significantly reduced the macrophage but not the SMC content. Cycloheximide-treated plaques showed signs of apoptosis (increased terminal deoxynucleotidyl transferase end labeling and fluorescein isothiocyanate-Val-Ala-dl-Asp(O-methyl)-fluoromethylketone labeling) that did not colocalize with SMCs. Organ chamber studies demonstrated that the functionality of SMCs and the endothelium were not influenced by cycloheximide treatment. All together, these findings demonstrate that cycloheximide decreases the macrophage load in atherosclerotic plaques by induction of apoptosis without changing SMC content or contractility.

Cell membranes and liposomes dissociate C-reactive protein (CRP) to form a new, biologically active structural intermediate: mCRP(m)

Emerging evidence indicates that C-reactive protein (CRP) has at least two conformationally distinct isoforms, i.e., pentameric CRP (pCRP) and monomeric CRP (mCRP or CRP subunit). Both CRP isoforms are proposed to play roles in inflammation and may participate in the pathogenesis of cardiovascular disease. However, the origin of mCRP in situ and the interplay between the two CRP isoforms under physiological/pathological circumstances remain elusive. Herein, by probing conformational alteration, neoepitope expression, and direct visualization using electron-microscopy, we have shown that calcium-dependent binding of pCRP to membranes, including liposomes and cell membranes, led to a rapid but partial structural change, producing molecules that express CRP subunit antigenicity but with retained native pentameric conformation. This hybrid molecule is herein termed mCRP(m). The formation of mCRP(m) was associated with significantly enhanced complement fixation. mCRP(m) can further detach from membrane to form the well-recognized mCRP isoform converted in solution (mCRP(s)) and exert potent stimulatory effects on endothelial cells. The membrane-induced pCRP dissociation not only provides a physiologically relevant scenario for mCRP formation but may represent an important mechanism for regulating CRP function.

Phospholipase A 2 –Modified Low-Density Lipoprotein Activates the Phosphatidylinositol 3-Kinase-Akt Pathway and Increases Cell Survival in Monocytic Cells

Objective— Monocyte survival is an important determinant in the development of atherosclerotic lesions. We investigated the influence of phospholipase A 2 -modified LDL (PLA-LDL), a pro-atherogenic factor, on activation of the pro-survival kinase Akt and cell death in monocytic cells.

Methods and Results— PLA-LDL induced robust phosphorylation and activation of Akt in THP1 cells. It also attenuated oxidative stress-induced cell death, an effect abolished by phosphatidylinositol 3-kinase (PI3K) inhibition. In addition, PLA-LDL increased survival of human monocytes. We noticed that lipid products derived from LDL phospholipolysis are mediators of PLA-LDL–induced Akt activation. Arachidonic acid, which is released on phospholipase treatment of LDL, induced Akt phosphorylation and increased cell survival, whereas lysophosphatidylcholine, another compound generated by LDL phospholipolysis, induced only transient Akt phosphorylation and was cytotoxic.

Conclusions— Our data indicate that PLA-LDL induces activation of the PI3K-Akt pathway and promotes monocytic cell survival, which may contribute to the pro-atherogenic effects of phospholipase A 2 -modified LDL.

Homozygous disruption of Pctp modulates atherosclerosis in apolipoprotein E-deficient mice

Phosphatidylcholine transfer protein (PC-TP) is a cytosolic phospholipid binding protein and a member of the steroidogenic acute regulatory-related transfer domain superfamily. Its tissue distribution includes liver and macrophages. PC-TP regulates hepatic lipid metabolism, and its absence in cholesterol-loaded macrophages is associated with reduced ATP binding cassette transporter A1-mediated lipid efflux and increased susceptibility to apoptosis induced by unesterified cholesterol. To explore a role for PC-TP in atherosclerosis, we prepared PC-TP-deficient/apolipoprotein E-deficient (Pctp(-/-)/Apoe(-/-)) mice and littermate Apoe(-/-) controls. At 16 weeks, atherosclerosis was increased in chow-fed male, but not female, Pctp(-/-)/Apoe(-/-) mice. This effect was associated with increases in plasma lipid concentrations. By contrast, no differences in atherosclerosis were observed between male or female Pctp(-/-)/Apoe(-/-) mice and Apoe(-/-) controls fed a Western-type diet for 16 weeks. At 24 weeks, atherosclerosis in chow-fed male Pctp(-/-)/Apoe(-/-) mice tended to be reduced in proportion to plasma cholesterol. The attenuation of atherosclerosis in female Pctp(-/-)/Apoe(-/-) mice fed chow or the Western-type diet for 24 weeks was not attributable to changes in plasma cholesterol or triglyceride concentrations. These findings suggest that PC-TP modulates the development of atherosclerosis, in part by regulating plasma lipid concentrations.

The inflammatory cytokine response of cholesterol-enriched macrophages is dampened by stimulated pinocytosis

Two features of advanced atherosclerotic lesions are large numbers of macrophages and a heightened state of inflammation. Some of the macrophages appear to be enriched with free cholesterol (FCMphis), and we have shown that this process induces the synthesis and secretion of inflammatory cytokines, including TNF-alpha and IL-6. However, lesions contain many other macrophages that are not FC-enriched (non-FCMphis). Therefore, we sought to understand how the interaction of these two populations of macrophages would influence the inflammatory response. We show here that non-FCMphis possess a robust ability to deplete TNF-alpha and IL-6 secreted by FCMphis. The mechanism involves enhanced pinocytic uptake and lysosomal degradation of the FCMphi-secreted cytokines by the non-FCMphis. The FCMphis contribute directly to this process by secreting pinocytosis-stimulatory factors that act on non-FCMphis but not on the FCMphis themselves. One of these pinocytosis-stimulatory factors is M-CSF, which is induced by a process involving cholesterol trafficking to the endoplasmic reticulum and signaling through PI-3K and ERK MAPK pathways. However, one or more other FCMphi-secreted factors are also required for stimulating pinocytosis in non-FCMphis. Thus, FCMphis secrete inflammatory cytokines as well as factors that promote the eventual pinocytosis and degradation of these cytokines by neighboring macrophages. This process may normally serve to prevent prolonged or disseminated effects of inflammatory cytokines during inflammation. Moreover, possible perturbation of stimulated pinocytosis during the progression of advanced atherosclerosis may contribute to the heightened inflammatory state of these lesions.

Regression of atherosclerosis by amlodipine via anti-inflammatory and anti-oxidative stress actions

We examined whether amlodipine, an L-type calcium channel blocker (CCB), has an inhibitory effect on oxidative stress and inflammatory response, and thereby atherosclerosis, in apolipoprotein E-deficient (ApoEKO) mice. Adult male ApoEKO mice (6 weeks of age) were fed a high-cholesterol diet (HCD) for 8 or 10 weeks with or without oral administration of amlodipine (3 mg/kg/day) for 10 weeks or for only the last 2 weeks of the HCD. After HCD feeding, atherosclerotic lesion formation, in situ superoxide production and nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase activity were evaluated in the proximal aorta. The expressions of NADPH oxidase subunits (p47(phox) and rac-1), monocyte chemoattractant protein-1 (MCP-1), intercellular adhesion molecule-1 (ICAM-1), and vascular cell adhesion molecule-1 (VCAM-1) were determined with immunohistochemistry and quantitative real-time reverse-transcription polymerase chain reaction. After 8 to 10 weeks of HCD administration to ApoEKO mice, marked atherosclerotic lesion formation was observed in the proximal aorta. In the atherosclerotic lesion, superoxide production, the expression of NADPH oxidase subunits, and NADPH oxidase activity were enhanced, and the expressions of MCP-1, ICAM-1, and VCAM-1 were increased. These changes were suppressed in mice that were treated with amlodipine for 10 weeks concomitant with HCD administration, with no significant change in blood pressure and plasma cholesterol level. We also observed that treatment with amlodipine for only the last 2 weeks regressed the atherosclerotic lesions with a decrease in oxidative stress and vascular inflammation. Inhibition of the atherosclerotic lesion area and lipid area in the proximal aorta by amlodipine was correlated with its inhibitory actions on oxidative stress, inflammation and the production of adhesive molecules. These results suggest that amlodipine not only inhibits atherosclerotic lesion formation, but also regresses atherosclerosis, and that these effects are at least partly due to inhibition of oxidative stress and inflammatory response.

Anti-inflammatory effects of short chain fatty acids in IFN-gamma-stimulated RAW 264.7 murine macrophage cells: involvement of NF-kappaB and ERK signaling pathways

The overactivation of macrophages causes abnormal cell death and chronic inflammatory diseases. Therefore, the modulation of macrophage-mediated cytotoxicity is expected to become a new therapeutic strategy for various inflammatory diseases. In this study, three types of short chain fatty acids (sodium butyrate (NaB), sodium phenylbutyrate (NaPB), sodium phenylacetate (NaPA)) were found to have anti-inflammatory effects in IFN-gamma-stimulated RAW 264.7 cells. They inhibited the expression of iNOS, TNF-alpha, and IL-6 induced by IFN-gamma, while they enhanced the expression of the anti-inflammatory cytokine, IL-10. Their potency as anti-inflammatory agents was in the order of NaB>NaPB>NaPA. Further mechanistic studies revealed these three agents to repress the DNA binding and transcriptional activities of NF-kappaB, which is an important modulator of inflammation. In addition, these agents repressed the IFN-gamma-induced ERK1/2 phosphorylation without affecting the Jak/STAT activities. The potency of NF-kappaB and ERK inhibition was also in the order of NaB>NaPB>NaPA. The results suggest that the NF-kappaB and ERK signaling pathways are at least in part involved in the anti-inflammatory activities of these SCFAs. Considering that SCFAs are normally present in the body and have few side effects, they might be promising agents for the prevention and/or treatment of various inflammatory diseases.

Sitosterol-containing lipoproteins trigger free sterol-induced caspase-independent death in ACAT-competent macrophages

Sitosterolemia is a disease characterized by very high levels of sitosterol and other plant sterols and premature atherothrombotic vascular disease. One theory holds that plant sterols can directly promote atherosclerosis, but the mechanism is not known. Unesterified, or "free," cholesterol (FC) is a potent inducer of macrophage death, which causes plaque necrosis, a precursor to atherothrombosis. FC-induced macrophage death, however, requires dysfunction of the sterol esterifying enzyme acyl-coenzyme A-cholesterol acyltransferase (ACAT), which likely occurs slowly during lesion progression. In contrast, plant sterols are relatively poorly esterified by ACAT, and so they may cause macrophage death and plaque necrosis in an accelerated manner. In support of this hypothesis, we show here that macrophages incubated with sitosterol-containing lipoproteins accumulate free sterols and undergo death in the absence of an ACAT inhibitor. As with FC loading, sitosterol-induced macrophage death requires sterol trafficking to the endoplasmic reticulum, and sitosterol-enriched endoplasmic reticulum membranes show evidence of membrane protein dysfunction. However, whereas FC induces caspase-dependent apoptosis through activation of the unfolded protein response and JNK, sitosterol-induced death is caspase-independent and involves neither the unfolded protein response nor JNK. Rather, cell death shows signs of necroptosis and autophagy and is suppressed by inhibitors of both processes. These data establish two new concepts. First, a relatively subtle change in sterol structure fundamentally alters the type of death program triggered in macrophages. Understanding the basis of this alteration should provide new insights into the molecular basis of death pathway signaling. Second, sitosterol-induced macrophage death does not require ACAT dysfunction and so may occur in an accelerated fashion. Pending future in vivo studies, this concept may provide at least one mechanism for accelerated plaque necrosis and atherothrombotic disease in patients with sitosterolemia.

Impaired development of atherosclerosis in hyperlipidemic Ldlr-/- and ApoE-/- mice transplanted with Abcg1-/- bone marrow

OBJECTIVE

The lungs of Abcg1-/- mice accumulate macrophage foam cells that contain high levels of unesterified and esterified cholesterol, consistent with a role for ABCG1 in facilitating the efflux of cholesterol from macrophages to high-density lipoprotein (HDL) and other exogenous sterol acceptors. Based on these observations, we investigated whether loss of ABCG1 affects foam cell deposition in the artery wall and the development of atherosclerosis.

METHODS AND RESULTS

Bone marrow from wild-type or Abcg1-/- mice was transplanted into Ldlr-/- or ApoE-/- mice. After administration of a high-fat/high-cholesterol diet, plasma and tissue lipid levels and atherosclerotic lesion size were quantified and compared. Surprisingly, transplantation of Abcg1-/- bone marrow cells resulted in a significant reduction in lesion size in both mouse models, despite the fact that lipid levels increased in the lung, spleen, and kidney. Lesions of Ldlr-/- mice transplanted with Abcg1-/- cells contained increased numbers of apoptotic cells. Consistent with this observation, in vitro studies demonstrated that Abcg1-/- macrophages were more susceptible to oxidized low-density lipoprotein (ox-LDL)-dependent apoptosis than Abcg1+/+ cells.

CONCLUSIONS

Diet-induced atherosclerosis is impaired when atherosclerotic-susceptible mice are transplanted with Abcg1-/- bone marrow. The demonstration that Abcg1-/- macrophages undergo accelerated apoptosis provides a mechanism to explain the decrease in the atherosclerotic lesions.

Importance of acyl-coenzyme A:cholesterol acyltransferase 1/2 dual inhibition for anti-atherosclerotic potency of pactimibe

Pactimibe sulfate, [7-(2,2-dimethylpropanamido)-4,6-dimethyl-1-octylindolin-5-yl]acetic acid hemisulfate, a novel Acyl-coenzyme A:cholesterol acyltransferase (ACAT) inhibitor, was investigated in vitro and in vivo to characterize its potential. Pactimibe exhibited dual inhibition for ACAT1 and ACAT2 (concentrations inhibiting 50% [IC50s] at micromolar levels) more potently than avasimibe. Kinetic analysis revealed pactimibe is a noncompetitive inhibitor of oleoyl-CoA (Ki value: 5.6 microM). Furthermore, pactimibe markedly inhibited cholesteryl ester formation (IC50: 6.7 microM) in human monocyte-derived macrophages, and inhibited copper-induced oxidation of low density lipoprotein more potently than probucol. Pactimibe exerted potent lipid-lowering and anti-atherosclerotic effects in atherogenic diet-fed hamsters. At doses of 3 and 10 mg/kg for 90 days, pactimibe decreased serum total cholesterol by 70% and 72%, and aortic fatty streak area by 79% and 95%, respectively. Despite similar cholesterol lowering, fatty streak area reduction was greater by 10 mg/kg. These results suggest that ACAT1/2 dual inhibitor pactimibe has anti-atherosclerotic potential beyond its plasma cholesterol-lowering activity.

A natural antibody to oxidized cardiolipin binds to oxidized low-density lipoprotein, apoptotic cells, and atherosclerotic lesions

OBJECTIVE

Cardiolipin (CL) is found in membranes of bacteria, in the inner membrane of mitochondria and in plasma low-density lipoprotein (LDL). Anticardiolipin antibodies (aCL) are associated with disease states, and we have suggested that many aCL bind to oxidized CL (oxCL) but not native CL. To determine the immunogenicity and origins of oxCL in vivo, we cloned a natural antibody to oxCL.

METHODS AND RESULTS

A monoclonal IgM antibody to oxCL (LRO1) was cloned from a nonimmunized LDLR-/- mouse. The V(H) sequence originated from the V(H)Gam3.8 germline with one nucleotide difference, and the Vkappa was 100% identical to Vkappa19-20 germline gene, making LRO1 a natural antibody. LRO1 bound specifically to oxCL and oxidized-LDL, but not to native CL or native LDL. LRO1 epitopes were demonstrated in apoptotic, but not in viable, Jurkat cells by flow cytometry, immunofluorescence and deconvolution microscopy. Human and rabbit atherosclerotic lesions contained LRO1 epitopes. Human LDL (n=113) showed LRO1 immunoreactivity, which correlated with aCL IgG titers (r=0.32, P=0.0004).

CONCLUSIONS

These data demonstrate that some aCL antibodies are highly conserved natural antibodies binding to oxCL in oxLDL, apoptotic cells, and atherosclerotic lesions. This suggests that oxCL is one of the pathogen-associated molecular patterns of innate immunity and gives insight into the pathogenic events of diseases with increased titers of aCL antibodies.

Minimally Oxidized LDL Offsets the Apoptotic Effects of Extensively Oxidized LDL and Free Cholesterol in Macrophages

OBJECTIVE

Lipid-loaded macrophage-derived foam cells populate atherosclerotic lesions and produce many pro-inflammatory and plaque-destabilizing factors. An excessive accumulation of extensively oxidized low-density lipoprotein (OxLDL) or free cholesterol (FC), both of which are believed to be major lipid components of macrophages in advanced lesions, rapidly induces apoptosis in macrophages. Indeed, there is evidence of macrophage death in lesions, but how the surviving macrophages avoid death induced by OxLDL, FC, and other factors is not known.

METHODS AND RESULTS

Minimally oxidized LDL (mmLDL), which is an early product of progressive LDL oxidation in atherosclerotic lesions, countered OxLDL-induced or FC-induced apoptosis and stimulated macrophage survival both in cell culture and in vivo. DNA fragmentation and caspase-3 activity in OxLDL-treated peritoneal macrophages were significantly reduced by coincubation with mmLDL. In a separate set of experiments, mmLDL significantly reduced annexin V binding to macrophages in which apoptosis was induced by FC loading. In both cellular models, mmLDL activated a pro-survival PI3K/Akt signaling pathway, and PI3K inhibitors, wortmannin and LY294002, eliminated the pro-survival effect of mmLDL. Immunohistochemical examination demonstrated phospho-Akt in murine atherosclerotic lesions.

CONCLUSIONS

Minimally oxidized LDL, an early form of oxidized LDL in atherosclerotic lesions, may contribute to prolonged survival of macrophage foam cells in lesions via a PI3K/Akt-dependent mechanism.

Axl, a receptor tyrosine kinase, mediates flow-induced vascular remodeling

Intima-media thickening (IMT) in response to hemodynamic stress is a physiological process that requires coordinated signaling among endothelial, inflammatory, and vascular smooth muscle cells (VSMC). Axl, a receptor tyrosine kinase, whose ligand is Gas6, is highly induced in VSMC after carotid injury. Because Axl regulates cell migration, phagocytosis and apoptosis, we hypothesized that Axl would play a role in IMT. Vascular remodeling in mice deficient in Axl (Axl(-/-)) and wild-type littermates (Axl(+/+)) was induced by ligation of the left carotid artery (LCA) branches maintaining flow via the left occipital artery. Both genotypes had similar baseline hemodynamic parameters and carotid artery structure. Partial ligation altered blood flow equally in both genotypes: increased by 60% in the right carotid artery (RCA) and decreased by 80% in the LCA. There were no significant differences in RCA remodeling between genotypes. However, in the LCA Axl(-/-) developed significantly smaller intima+media compared with Axl(+/+) (31+/-4 versus 42+/-6x10(-6) microm3, respectively). Quantitative immunohistochemistry of Axl(-/-) LCA showed increased apoptosis compared with Axl(+/+) (5-fold). As expected, p-Akt was decreased in Axl(-/-), whereas there was no difference in Gas6 expression. Cell composition also changed significantly, with increases in CD45+ cells and decreases in VSMC, macrophages, and neutrophils in Axl(-/-) compared with Axl(+/+). These data demonstrate an important role for Axl in flow-dependent remodeling by regulating vascular apoptosis and vascular inflammation.

Pactimibe stabilizes atherosclerotic plaque through macrophage acyl-CoA:cholesterol acyltransferase inhibition in WHHL rabbits

Novel acyl coenzyme A:cholesterol acyltransferase (ACAT) inhibitor pactimibe was administered as the sulfate salt form to 3-month-old homozygous Watanabe heritable hyperlipidemic (WHHL) rabbits at doses of 0, 10, or 30 mg/kg for 32 weeks. Pactimibe (10 and 30 mg/kg) tended to reduce intimal thickening in thoracic aortic lesions (294+/-39 and 276+/-32 microm, respectively, versus 313+/-37 microm control), histopathological examination revealing significantly increased smooth muscle cell area (12.0+/-0.9% and 12.3+/-0.5%, P<0.05, respectively, versus 9.7+/-0.8% control), significantly increased collagen fiber area (20.5+/-1.2% and 31.0+/-1.3%, P<0.05, respectively, versus 16.2+/-1.0% control), and tended to reduce macrophage infiltration (6.0+/-1.1% and 4.6+/-1.0%, respectively, versus 7.0+/-1.3% control). Pactimibe dose-dependently reduced cholesteryl ester content in thoracic and abdominal aortic lesions, and reduced free cholesterol content in the aorta versus control. Although pactimibe did not alter serum cholesterol levels in WHHL rabbits, it stabilized vulnerable plaque characterized with reduced cholesteryl ester content, enriched collagen fibers and increased smooth muscle cells, indicating potential as a treatment strategy for coronary heart disease.

Macrophage insulin receptor deficiency increases ER stress-induced apoptosis and necrotic core formation in advanced atherosclerotic lesions

Insulin resistance in diabetes and metabolic syndrome is thought to increase susceptibility to atherosclerotic cardiovascular disease, but the underlying mechanisms are poorly understood. To evaluate the possibility that decreased insulin signaling in macrophage foam cells might worsen atherosclerosis, Ldlr(-/-) mice were transplanted with insulin receptor Insr(+/+) or Insr(-/-) bone marrow. Western diet-fed Insr(-/-) recipients developed larger, more complex lesions with increased necrotic cores and increased numbers of apoptotic cells. Insr(-/-) macrophages showed diminished Akt phosphorylation and an augmented ER stress response, leading to induction of scavenger receptor A and increased apoptosis when challenged with cholesterol loading or nutrient deprivation. These studies suggest that defective insulin signaling and reduced Akt activity impair the ability of macrophages to deal with ER stress-induced apoptosis within atherosclerotic plaques.

Cholesterol-induced Apoptotic Macrophages Elicit an Inflammatory Response in Phagocytes, Which Is Partially Attenuated by the Mer Receptor

Macrophage apoptosis and the ability of phagocytes to clear these apoptotic cells are important processes in advanced atherosclerosis. Phagocytic clearance not only disposes of dead cells but usually elicits an anti-inflammatory response. To study this process in a model of advanced lesional macrophage death, macrophages rendered apoptotic by free cholesterol loading (FC-AMs) were incubated briefly with fresh macrophages ("phagocytes"). FC-AMs were promptly ingested by the phagocytes, which was dependent upon actin polymerization and the phagocyte Mer receptor. Surprisingly, this brief exposure to FC-AMs triggered a modest proinflammatory response in the phagocytes: tumor necrosis factor-alpha (TNF-alpha) and interleukin (IL)-1beta were induced, whereas the levels of transforming growth factor-beta and IL-10 were not increased. This response required cell contact between the FC-AMs and phagocytes but not FC-AM ingestion. TNF-alpha and IL-1beta induction required one or more proteins on the FC-AM surface and was dependent on signaling through extracellular signal-regulated kinase-1/2 mitogen-activated protein kinase and nuclear factor-kappaB in the phagocytes. TNF-alpha production was markedly greater when Mer-defective phagocytes were used, indicating that Mer attenuated the inflammatory response. Interestingly, a more typical anti-inflammatory response was elicited when phagocytes were exposed to macrophages rendered apoptotic by oxidized low density lipoprotein or UV radiation. Thus, the proinflammatory milieu of advanced atherosclerotic lesions may be promoted, or at least not dampened, by contact between FC-induced apoptotic macrophages and neighboring phagocytes prior to apoptotic cell ingestion.

Role of cholesterol and lipid organization in disease

Membrane lipids are essential for biological functions ranging from membrane trafficking to signal transduction. The composition of lipid membranes influences their organization and properties, so it is not surprising that disorders in lipid metabolism and transport have a role in human disease. Significant recent progress has enhanced our understanding of the molecular and cellular basis of lipid-associated disorders such as Tangier disease, Niemann-Pick disease type C and atherosclerosis. These insights have also led to improved understanding of normal physiology.