Carrie L. Welch

ATP-Binding Cassette Transporters and HDL Suppress Hematopoietic Stem Cell Proliferation

Inhibiting Leukocytosis Leukocytosis—an elevated white blood cell count—contributes by unknown mechanisms to the pathogenesis of atherosclerosis and associated coronary heart disease. Now, Yvan-Charvet et al. (p. 1689, published online 20 May; see the Perspective by Hansson and Björkholm) show that the adenosine triphosphate–binding cassette transporters ABCA1 and ABCG1 are critical suppressors of atherosclerosis-associated leukocytosis. Mice deficient in both transporters in blood-producing hematopoietic cells possessed increased levels of hematopoietic stem and multipotential progenitor cells and accelerated atherosclerosis. ABCA1 and ABGA1 protect against atherosclerosis by promoting cholesterol efflux from cholesterol-laden macrophage foam cells to lipid-poor high-density lipoprotein (HDL) and apolipoprotein A-1. The leukocytosis and atherosclerosis in ABCA1- and ABG1-deficient mice were reversed in the presence of high amounts of HDL. Thus, signaling already known to inhibit atherosclerosis by reducing cholesterol in atherosclerotic plaques also reduces atherosclerosis-associated leukocytosis. Pathways that reduce cholesterol in atherosclerosis also suppress increased immune cell numbers associated with the disease. Elevated leukocyte cell numbers (leukocytosis), and monocytes in particular, promote atherosclerosis; however, how they become increased is poorly understood. Mice deficient in the adenosine triphosphate–binding cassette (ABC) transporters ABCA1 and ABCG1, which promote cholesterol efflux from macrophages and suppress atherosclerosis in hypercholesterolemic mice, displayed leukocytosis, a transplantable myeloproliferative disorder, and a dramatic expansion of the stem and progenitor cell population containing Lin–Sca-1+Kit+ (LSK) in the bone marrow. Transplantation of Abca1–/– Abcg1–/– bone marrow into apolipoprotein A-1 transgenic mice with elevated levels of high-density lipoprotein (HDL) suppressed the LSK population, reduced leukocytosis, reversed the myeloproliferative disorder, and accelerated atherosclerosis. The findings indicate that ABCA1, ABCG1, and HDL inhibit the proliferation of hematopoietic stem and multipotential progenitor cells and connect expansion of these populations with leukocytosis and accelerated atherosclerosis.

Combined deficiency of ABCA1 and ABCG1 promotes foam cell accumulation and accelerates atherosclerosis in mice

HDLs protect against the development of atherosclerosis, but the underlying mechanisms are poorly understood. HDL and its apolipoproteins can promote cholesterol efflux from macrophage foam cells via the ATP-binding cassette transporters ABCA1 and ABCG1. Experiments addressing the individual roles of ABCA1 and ABCG1 in the development of atherosclerosis have produced mixed results, perhaps because of compensatory upregulation in the individual KO models. To clarify the role of transporter-mediated sterol efflux in this disease process, we transplanted BM from Abca1(-/-)Abcg1(-/-) mice into LDL receptor-deficient mice and administered a high-cholesterol diet. Compared with control and single-KO BM recipients, Abca1(-/-)Abcg1(-/-) BM recipients showed accelerated atherosclerosis and extensive infiltration of the myocardium and spleen with macrophage foam cells. In experiments with isolated macrophages, combined ABCA1 and ABCG1 deficiency resulted in impaired cholesterol efflux to HDL or apoA-1, profoundly decreased apoE secretion, and increased secretion of inflammatory cytokines and chemokines. In addition, these cells showed increased apoptosis when challenged with free cholesterol or oxidized LDL loading. These results suggest that the combined effects of ABCA1 and ABCG1 in mediating macrophage sterol efflux are central to the antiatherogenic properties of HDL.

Increased inflammatory gene expression in ABC transporter deficient macrophages: free cholesterol accumulation, increased signaling via Toll-like receptors and neutrophil infiltration of atherosclerotic lesions

Background— Two macrophage ABC transporters, ABCA1 and ABCG1, have a major role in promoting cholesterol efflux from macrophages. Peritoneal macrophages deficient in ABCA1, ABCG1, or both show enhanced expression of inflammatory and chemokine genes. This study was undertaken to elucidate the mechanisms and consequences of enhanced inflammatory gene expression in ABC transporter–deficient macrophages. Methods and Results— Basal and lipopolysaccharide-stimulated thioglycollate-elicited peritoneal macrophages showed increased inflammatory gene expression in the order Abca1−/−Abcg1−/−>Abcg1−/−>Abca1−/−>wild-type. The increased inflammatory gene expression was abolished in macrophages deficient in Toll-like receptor 4 (TLR4) or MyD88/TRIF. TLR4 cell surface concentration was increased in Abca1−/−Abcg1−/−>Abcg1−/−> Abca1−/−> wild-type macrophages. Treatment of transporter-deficient cells with cyclodextrin reduced and cholesterol-cyclodextrin loading increased inflammatory gene expression. Abca1−/−Abcg1− bone marrow–derived macrophages showed enhanced inflammatory gene responses to TLR2, TLR3, and TLR4 ligands. To assess in vivo relevance, we injected intraperitoneally thioglycollate in Abcg1−/− bone marrow–transplanted, Western diet–fed, Ldlr-deficient mice. This resulted in a profound inflammatory infiltrate in the adventitia and necrotic core region of atherosclerotic lesions, consisting primarily of neutrophils. Conclusions— The results suggest that high-density lipoprotein and apolipoprotein A-1 exert anti-inflammatory effects by promoting cholesterol efflux via ABCG1 and ABCA1 with consequent attenuation of signaling via Toll-like receptors. In response to a peripheral inflammatory stimulus, atherosclerotic lesions containing Abcg1−/− macrophages experience an inflammatory “echo,” suggesting a possible mechanism of plaque destabilization in subjects with low high-density lipoprotein levels.

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

Leukocytosis is associated with increased cardiovascular disease risk in humans and develops in hypercholesterolemic atherosclerotic animal models. Leukocytosis is associated with the proliferation of hematopoietic stem and multipotential progenitor cells (HSPCs) in mice with deficiencies of the cholesterol efflux-promoting ABC transporters ABCA1 and ABCG1 in BM cells. Here, we have determined the role of endogenous apolipoprotein-mediated cholesterol efflux pathways in these processes. In Apoe⁻/⁻ mice fed a chow or Western- type diet, monocytosis and neutrophilia developed in association with the proliferation and expansion of HSPCs in the BM. In contrast, Apoa1⁻/⁻ mice showed no monocytosis compared with controls. ApoE was found on the surface of HSPCs, in a proteoglycan-bound pool, where it acted in an ABCA1- and ABCG1-dependent fashion to decrease cell proliferation. Accordingly, competitive BM transplantation experiments showed that ApoE acted cell autonomously to control HSPC proliferation, monocytosis, neutrophilia, and monocyte accumulation in atherosclerotic lesions. Infusion of reconstituted HDL and LXR activator treatment each reduced HSPC proliferation and monocytosis in Apoe⁻/⁻ mice. These studies suggest a specific role for proteoglycanbound ApoE at the surface of HSPCs to promote cholesterol efflux via ABCA1/ABCG1 and decrease cell proliferation, monocytosis, and atherosclerosis. Although endogenous apoA-I was ineffective, pharmacologic approaches to increasing cholesterol efflux suppressed stem cell proliferative responses.

ABCG1 and HDL protect against endothelial dysfunction in mice fed a high-cholesterol diet

Plasma HDL levels are inversely related to the incidence of atherosclerotic disease. Some of the atheroprotective effects of HDL are likely mediated via preservation of EC function. Whether the beneficial effects of HDL on ECs depend on its involvement in cholesterol efflux via the ATP-binding cassette transporters ABCA1 and ABCG1, which promote efflux of cholesterol and oxysterols from macrophages, has not been investigated. To address this, we assessed endothelial function in Abca1(-/-), Abcg1(-/-), and Abca1(-/-)Abcg1(-/-) mice fed either a high-cholesterol diet (HCD) or a Western diet (WTD). Non-atherosclerotic arteries from WTD-fed Abcg1(-/-) and Abca1(-/-)Abcg1(-/-) mice exhibited a marked decrease in endothelium-dependent vasorelaxation, while Abca1(-/-) mice had a milder defect. In addition, eNOS activity was reduced in aortic homogenates generated from Abcg1(-/-) mice fed either a HCD or a WTD, and this correlated with decreased levels of the active dimeric form of eNOS. More detailed analysis indicated that ABCG1 was expressed primarily in ECs, and that these cells accumulated the oxysterol 7-ketocholesterol (7-KC) when Abcg1(-/-) mice were fed a WTD. Consistent with these data, ABCG1 had a major role in promoting efflux of cholesterol and 7-KC in cultured human aortic ECs (HAECs). Furthermore, HDL treatment of HAECs prevented 7-KC-induced ROS production and active eNOS dimer disruption in an ABCG1-dependent manner. Our data suggest that ABCG1 and HDL maintain EC function in HCD-fed mice by promoting efflux of cholesterol and 7-oxysterols and preserving active eNOS dimer levels.

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

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

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-ABCDKO mice) but not in hematopoietic stem or progenitor populations. MAC-ABCDKO 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-ABCDKO 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-ABCDKO 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.

Complex genetic control of HDL levels in mice in response to an atherogenic diet. Coordinate regulation of HDL levels and bile acid metabolism.

Inbred strains of mice differ in susceptibility to atherogenesis when challenged with a high fat, high cholesterol diet containing 0.5% cholic acid. Studies of recombinant inbred (RI) strains derived from the susceptible strain C57BL/6J (B6) and the resistant strains C3H/HeJ (C3H) and BALB/cJ have revealed an association between fatty streak lesion size and a decrease in high density lipoprotein (HDL) levels on the diet. To better understand the genetic factors contributing to HDL metabolism and atherogenesis in response to the diet, we studied mice derived from an intercross between B6 and C3H using a complete linkage map approach. A total of 185 female progeny were typed for 134 genetic markers spanning the mouse genome, resulting in an average interval of about 10 cM between markers. A locus on distal chromosome 1 containing the apolipoprotein AII gene was linked to HDL-cholesterol levels on both the chow and the atherogenic diets, but this locus did not contribute to the decrease in HDL-cholesterol in response to the diet. At least three distinct genetic loci, on chromosomes 3, 5, and 11, exhibited evidence of linkage to a decrease in HDL-cholesterol after a dietary challenge. Since a bile acid (cholic acid) is required for the diet induced changes in HDL levels and for atherogenesis in these strains, we examined cholesterol-7-alpha hydroxylase (C7AH) expression. Whereas B6 mice exhibited a large decrease in C7AH mRNA levels in response to the diet, C3H showed an increase. Among the intercross mice, multiple loci contributed to the regulation of C7AH mRNA levels in response to the diet, the most notable of which coincided with the loci on chromosomes 3, 5, and 11 controlling HDL levels in response to the diet. None of these loci were linked to the C7AH structural gene which we mapped to proximal chromosome 4. These studies reveal coordinate regulation of C7AH expression and HDL levels, and they indicate that the genetic factors controlling HDL levels are more complex than previously suggested by studies of RI strains. Furthermore, we observed that two of the loci for C7AH expression contributed to differences in gallstone formation between these strains.

Deficiency of ABCA1 and ABCG1 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-ABCDKO mice) but not in hematopoietic stem or progenitor populations. MAC-ABCDKO 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-ABCDKO 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-ABCDKO 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.

Antiproliferative effect of the garlic compound S-allyl cysteine on human neuroblastoma cells in vitro

A variety of compounds derived from garlic bulbs have been shown in animal systems to possess anticancer properties. However, little information is available regarding the effectiveness of garlic in the prevention or treatment of human cancers. In the current study, we have assessed the ability of S-allyl cysteine (SAC), a derivative of aged garlic extract, to affect the proliferation and differentiation of LA-N-5 human neuroblastoma cells in vitro. Time-and dose-dependent inhibition of cell grow was observed in cultures treated with SAC for at least 2 days, with a half-maximal response at approximately 600 micrograms/ml. SAC treatment was unable to induce differentiation in neuroblastoma cells as assessed by morphological, biochemical and molecular markers. In addition, SAC was unable to potentiate the effects of retinoic acid and 8-bromo-cyclic AMP, agents known to promote differentiation of LA-N-5 cells. Our results indicate that SAC can inhibit human neuroblastoma cell growth in vitro. However, the apparent inability of this compound to induce differentiation may limit its therapeutic potential.